docs(lint): correct the Vale scalar, size-ceiling and release-gate claims

Four claims in shipped agent-facing docs did not match verified behaviour.
These are read as ground truth by agents in other repos, so each was
reproduced against vale 3.15.2 before rewriting:

- CONTEXT.md and `vale-config/SKILL.md` said both `>` and `|` block scalars
  break the description scope. `|` does not — it lints normally and fires every
  alert, while `>` yields zero. An agent following the old text would rewrite a
  working `|` description into a plain multi-line scalar, which genuinely does
  break, inverting the intended remediation. Both now name the forms that do
  break and state that `|` does not.
- CONTEXT.md and ADR-0013 described the size hook as failing only above 500
  lines, omitting the 2900-word gate it also enforces. Both now describe the
  pair and state that `validate.sh` checks the same two.
- ADR-0014 recorded an accepted residual — a wholesale `assets/` deletion going
  unflagged — that commit 14c2c91 closed. Left as the point-in-time record and
  amended with an update describing the union-with-tag-manifest mechanism,
  following the amendment precedent in ADR-0005.
- `vale-config/SKILL.md` asserted a fresh `.vale.ini` fails until `vale sync`
  runs, contradicting its own note that built-in styles need no download. The
  claim is now scoped to package styles; this repo's two configs declare no
  packages and lint clean with zero syncs.

Also repoints AGENTS.md at the seven `gitea:*` skills — the `bin:gitea` route
it named no longer exists.

Refs: #85

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

@@ -19,10 +19,10 @@ metadata:
 
 ## Gotchas
 
-- Installing the `vale` binary installs no styles. A fresh `.vale.ini` with `BasedOnStyles` set will fail or find nothing until `vale sync` runs and downloads the `Packages` it declares.
+- Installing the `vale` binary installs no styles, but only *package* styles need fetching. A fresh `.vale.ini` naming a style in `BasedOnStyles` that is declared in `Packages` will fail or find nothing until `vale sync` downloads it. A built-in style (`Vale`) or a style whose YAML rule files are already committed under `StylesPath` lints immediately, with no `Packages` entry and no sync.
 - `.vale.ini` is order-sensitive: global (core) settings first, then the optional `[formats]` section, then glob sections (`[*]`, `[*.md]`, …). Settings in a glob section only apply to files matching that glob.
 - `Packages` (top-level, fetched by `vale sync`) and `BasedOnStyles` (per-glob, activates) are separate keys — a style only lints files once it's in both. This is the step people forget.
-- A rule scoped to `text.frontmatter.<key>` (e.g. `text.frontmatter.description`) only reliably matches when that field's value is a single physical line. If it's a YAML block scalar (`>`/`|`) spanning 2+ physical lines, the scope silently stops matching — no error, just 0 findings — confirmed against Vale 3.15.2. Verify with a deliberately-bad multi-line fixture before trusting a frontmatter-scoped rule in production; if the field is commonly authored as a multi-line block scalar, flatten it to one line ahead of the `vale` call rather than relying on the scope alone.
+- A rule scoped to `text.frontmatter.<key>` (e.g. `text.frontmatter.description`) matches reliably when that field's value is a single physical line, and breaks on most — not all — multi-line forms. Confirmed against Vale 3.15.2 with a deliberately-bad fixture: a `>` folded block scalar, plain (unquoted) continuation lines, and single- or double-quoted multi-line scalars each yield 0 findings and exit 0, silently and with no error; a `|` literal block scalar spanning the same 2+ lines lints normally and exits 1. Do not assume `|` and `>` behave alike — reproduce both against your own config before trusting a frontmatter-scoped rule in production. If the field is commonly authored in one of the broken forms, flatten it to one physical line ahead of the `vale` call rather than relying on the scope alone.
 
 ## Setup workflow