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

AGENTS.md

@@ -13,7 +13,7 @@ This repo dogfoods its own plugins. Before shelling out to git, gitea, or lint t
 
 - Commits, branches, history, worktrees, remotes → `git:git-commits`, `git:git-branches`, `git:git-history`, `git:git-worktrees`, `git:git-remotes`
 - Pre-commit hook install/config/troubleshooting → `git:pc-run` / `git:pc-author`
-- Issues, PRs, labels, milestones → `bin:gitea`
+- Issues, PRs, labels, milestones → `gitea:gitea-issues`, `gitea:gitea-prs`, `gitea:gitea-labels-milestones`; also `gitea:gitea-branches`, `gitea:gitea-files`, `gitea:gitea-releases`, or `gitea:gitea-workflow` when the domain is ambiguous
 - Vale prose linting → `lint:vale-config` / `lint:vale-run`
 - This repo's own AGENTS.md → `core:agentsmd-author` / `core:agentsmd-audit`