fix(lint): restore the glob-scoping rule and warn about the MDX invocation kill

vale-config deleted "settings in a glob section only apply to files matching that
glob" — the single most common Vale misconfiguration. Nothing in either lint skill
told an agent that BasedOnStyles under `[*.md]` governs `.md` only; the nearest
survivor was a parenthetical in a reference file the skill opens only for the full
field listing. Restored to the always-loaded body, at the point of use.

vale-config also never mentioned `[formats]` or MDX at all, leaving the whole
decision in that same reference file. Verified against Vale 3.15.2: an unmapped
`.mdx` covered by one of your globs is `E100 [lintMDX]`, exit 2, and every other
file in that invocation produces no output whatsoever. Because the failure is
invocation-wide rather than per-file, it belongs in the Gotchas.

The crash requires a glob that actually covers `.mdx` — under `[*.md]` the file is
skipped and nothing fails — so the Gotcha states that condition rather than the
broader claim that any `.mdx` in the tree kills the run.

The mapping-versus-mdx2vast attribution disagreed across the two skills: vale-run
said `[formats] mdx = md` is "what vale-config recommends" while vale-config
presented it as a bare either/or. Fixed on the source side by having vale-config
actually recommend it and say why, which makes vale-run's existing sentence true
with no edit to vale-run.

Refs: #117

plugins/lint/.apm/skills/vale-config/SKILL.md

@@ -20,6 +20,7 @@ metadata:
 - `vale sync` alone does not clear that `E100`. Sync fetches only what the top-level `Packages` key declares, so against a `BasedOnStyles`-only name it reports `Synced 0 package(s)` and exits 0, fetching nothing. Add the style to `Packages`, then sync. A style lints only once it is in both keys — and the reverse case is silent, exiting 0.
 - Only *package* styles need fetching: built-in `Vale`, and any style whose YAML is already committed under `StylesPath`, lint with no `Packages` entry and no sync.
 - `.vale.ini` order is enforced, not stylistic: put core settings first, then `[formats]`, then glob sections. A core setting (`StylesPath`, `MinAlertLevel`, `Vocab`, `IgnoredScopes`, `SkippedScopes`) written below a `[glob]` header is a hard error — `E201 ... 'StylesPath' is a core option; it should be defined above any syntax-specific options`, exit 2, nothing linted. `Packages` is the exception, and the worse one: below a glob header it is accepted with no error, then ignored — `vale sync` reports `Synced 0 package(s)` and downloads nothing.
+- An `.mdx` file covered by one of your globs takes the whole run down unless `[formats]` maps it. Vale 3.15.2 has no built-in MDX support: unmapped, it shells out to an external `mdx2vast` binary, and with that absent from `PATH` the invocation dies on `E100 [lintMDX] Runtime error / mdx2vast not found`, exit 2 — every other file in the same command goes unlinted, with no output of its own. Default to the mapping — a `[formats]` section holding `mdx = md`, above the glob sections — which needs nothing installed; `npm install -g mdx2vast` is the alternative. The choice also inverts the inline-suppression syntax `vale-run` uses, so record which one the config took.
 - A rule scoped to `text.frontmatter.<key>` silently matches nothing when the field spans multiple lines in most YAML forms. If you scope a rule to frontmatter, read `references/configuration-reference.md` first.
 
 ## Setup workflow
@@ -35,6 +36,8 @@ metadata:
   BasedOnStyles = Vale
   ```
   `Vale` here is the built-in style (`Vale.Spelling`, `Vale.Terms`, `Vale.Avoid`, `Vale.Repetition`): no `Packages` entry and no `vale sync`. It still needs the `StylesPath` directory to exist — declare `StylesPath = styles` without creating `styles/` and even a `Vale`-only config dies with `E201 ... The path '...' does not exist`, exit 2. That is why the previous step creates the directory.
+
+  **Settings in a glob section only apply to files matching that glob.** `BasedOnStyles` under `[*.md]` governs `.md` and nothing else: a `.mdx`, `.rst` or `.txt` in the same tree has no style active, is skipped without being counted, and a run over only such files reports `0 files` and exits 0 — indistinguishable from clean. Give every extension you mean to lint a glob that covers it.
 - [ ] **Add third-party styles** (optional) by declaring them in `Packages`, then activating them in the same or another glob's `BasedOnStyles`:
   ```ini
   Packages = Google, write-good

plugins/lint/.apm/skills/vale-config/references/configuration-reference.md

@@ -25,7 +25,7 @@ Map an unrecognized extension onto a supported one so Vale lints it with the rig
 mdx = md
 ```
 
-`mdx` is the case that matters, because Vale 3.15.2 has no built-in MDX support. The mapping above is not cosmetic: it is what lets `.mdx` files lint with nothing else installed. Leave it out and Vale takes the native MDX path, which shells out to an external `mdx2vast` binary — absent from `PATH`, the run dies with `E100 [lintMDX] Runtime error / mdx2vast not found`, exit 2, and every other file in the same invocation goes unlinted too. Install it with `npm install -g mdx2vast` or take the mapping.
+`mdx` is the case that matters, because Vale 3.15.2 has no built-in MDX support. The mapping above is not cosmetic: it is what lets `.mdx` files lint with nothing else installed. Leave it out and Vale takes the native MDX path, which shells out to an external `mdx2vast` binary — absent from `PATH`, the run dies with `E100 [lintMDX] Runtime error / mdx2vast not found`, exit 2, and every other file in the same invocation goes unlinted too. Take the mapping: it is this skill's recommended default, because it needs nothing installed, and it is the branch `vale-run` assumes when it documents inline suppressions. Install `mdx2vast` (`npm install -g mdx2vast`) only when something else in the toolchain already needs the native MDX parser.
 
 The choice also decides the inline-suppression syntax, and it is inverted between the two: mapped to `md`, `.mdx` takes Markdown's `<!-- vale off -->`; native, it takes `{/* vale off */}`. `vale-run`'s `references/troubleshooting.md` carries the verified matrix.
 

plugins/lint/skills/vale-config/SKILL.md

@@ -20,6 +20,7 @@ metadata:
 - `vale sync` alone does not clear that `E100`. Sync fetches only what the top-level `Packages` key declares, so against a `BasedOnStyles`-only name it reports `Synced 0 package(s)` and exits 0, fetching nothing. Add the style to `Packages`, then sync. A style lints only once it is in both keys — and the reverse case is silent, exiting 0.
 - Only *package* styles need fetching: built-in `Vale`, and any style whose YAML is already committed under `StylesPath`, lint with no `Packages` entry and no sync.
 - `.vale.ini` order is enforced, not stylistic: put core settings first, then `[formats]`, then glob sections. A core setting (`StylesPath`, `MinAlertLevel`, `Vocab`, `IgnoredScopes`, `SkippedScopes`) written below a `[glob]` header is a hard error — `E201 ... 'StylesPath' is a core option; it should be defined above any syntax-specific options`, exit 2, nothing linted. `Packages` is the exception, and the worse one: below a glob header it is accepted with no error, then ignored — `vale sync` reports `Synced 0 package(s)` and downloads nothing.
+- An `.mdx` file covered by one of your globs takes the whole run down unless `[formats]` maps it. Vale 3.15.2 has no built-in MDX support: unmapped, it shells out to an external `mdx2vast` binary, and with that absent from `PATH` the invocation dies on `E100 [lintMDX] Runtime error / mdx2vast not found`, exit 2 — every other file in the same command goes unlinted, with no output of its own. Default to the mapping — a `[formats]` section holding `mdx = md`, above the glob sections — which needs nothing installed; `npm install -g mdx2vast` is the alternative. The choice also inverts the inline-suppression syntax `vale-run` uses, so record which one the config took.
 - A rule scoped to `text.frontmatter.<key>` silently matches nothing when the field spans multiple lines in most YAML forms. If you scope a rule to frontmatter, read `references/configuration-reference.md` first.
 
 ## Setup workflow
@@ -35,6 +36,8 @@ metadata:
   BasedOnStyles = Vale
   ```
   `Vale` here is the built-in style (`Vale.Spelling`, `Vale.Terms`, `Vale.Avoid`, `Vale.Repetition`): no `Packages` entry and no `vale sync`. It still needs the `StylesPath` directory to exist — declare `StylesPath = styles` without creating `styles/` and even a `Vale`-only config dies with `E201 ... The path '...' does not exist`, exit 2. That is why the previous step creates the directory.
+
+  **Settings in a glob section only apply to files matching that glob.** `BasedOnStyles` under `[*.md]` governs `.md` and nothing else: a `.mdx`, `.rst` or `.txt` in the same tree has no style active, is skipped without being counted, and a run over only such files reports `0 files` and exits 0 — indistinguishable from clean. Give every extension you mean to lint a glob that covers it.
 - [ ] **Add third-party styles** (optional) by declaring them in `Packages`, then activating them in the same or another glob's `BasedOnStyles`:
   ```ini
   Packages = Google, write-good

plugins/lint/skills/vale-config/references/configuration-reference.md

@@ -25,7 +25,7 @@ Map an unrecognized extension onto a supported one so Vale lints it with the rig
 mdx = md
 ```
 
-`mdx` is the case that matters, because Vale 3.15.2 has no built-in MDX support. The mapping above is not cosmetic: it is what lets `.mdx` files lint with nothing else installed. Leave it out and Vale takes the native MDX path, which shells out to an external `mdx2vast` binary — absent from `PATH`, the run dies with `E100 [lintMDX] Runtime error / mdx2vast not found`, exit 2, and every other file in the same invocation goes unlinted too. Install it with `npm install -g mdx2vast` or take the mapping.
+`mdx` is the case that matters, because Vale 3.15.2 has no built-in MDX support. The mapping above is not cosmetic: it is what lets `.mdx` files lint with nothing else installed. Leave it out and Vale takes the native MDX path, which shells out to an external `mdx2vast` binary — absent from `PATH`, the run dies with `E100 [lintMDX] Runtime error / mdx2vast not found`, exit 2, and every other file in the same invocation goes unlinted too. Take the mapping: it is this skill's recommended default, because it needs nothing installed, and it is the branch `vale-run` assumes when it documents inline suppressions. Install `mdx2vast` (`npm install -g mdx2vast`) only when something else in the toolchain already needs the native MDX parser.
 
 The choice also decides the inline-suppression syntax, and it is inverted between the two: mapped to `md`, `.mdx` takes Markdown's `<!-- vale off -->`; native, it takes `{/* vale off */}`. `vale-run`'s `references/troubleshooting.md` carries the verified matrix.