fix(lint): correct four Vale behaviours the skills described wrongly

Each of these would send a user down a path Vale does not support:

Core options placed under a glob header are not scoped to that glob — Vale
rejects them with E201, so the guidance to nest them produced a config that
will not load. The built-in `Vale` style is compiled in, but Vale still
requires StylesPath to exist on disk before it will run, so the "no StylesPath
needed" shortcut fails. The MDX guidance was inverted: under `[formats]
mdx = md` the mapping is what makes MDX lint at all, and it needs the mdx2vast
prerequisite that was never mentioned. And a spelling rule's `ignore` paths
resolve against StylesPath, not against the rule file's own directory, so the
documented relative paths silently matched nothing.

plugins/lint/skills/vale-run/references/sources.md

@@ -7,3 +7,11 @@
 - **Research doc:** plugins/lint/docs/research/docs/vale/sources.md
 - **Contributing files:** SKILL.md, references/troubleshooting.md
 - **Status:** `extracted`
+
+## house-vale-3-15-2-repro
+
+- **URL:** (house-verified — reproduced locally against the `vale` binary, not an external source)
+- **Description:** Behaviour of Vale 3.15.2 established by running it against purpose-built fixtures in this repo, where vale.sh documents nothing or documents it wrongly: `.mdx` has no built-in support and needs either `[formats] mdx = md` or an external `mdx2vast` binary (absent, the whole invocation exits 2 with `E100 [lintMDX]`), the inline-suppression form inverts between those two configurations, the `spelling` check's `ignore` paths resolve against `StylesPath` or the working directory but never against the rule file's own directory and fail silently when they resolve nowhere, `ls-config` reports styles and paths but never rules, and the `text.frontmatter.<key>` scope matrix across multi-line YAML forms.
+- **Research doc:** none — house-verified reproduction, not part of the plugin's research corpus (no `plugins/lint/docs/research/` topic file backs this entry)
+- **Contributing files:** SKILL.md, references/troubleshooting.md
+- **Status:** `extracted`