fix(lint): tighten the SKILL.md word ceiling to 2770

MAX_WORDS=2900 was calibrated to the corpus median density and carried no
margin: at the densest observed 7.22 chars/word (~1.81 tokens/word) it permits
~5,240 tokens against the 5,000 it proxies for. 2770 holds the worst observed
density under the ceiling. The largest SKILL.md is 2,489 words, so the change
costs nothing today — 281 words of margin — and the header comment now argues
the new calibration rather than swapping the digits.

Both enforcement points move together, and a new test asserts they agree, since
a SKILL.md passing its own audit while the commit hook blocks it is the
disagreement this pair exists to prevent.

CONTEXT.md is deliberately left ungated: it is 2,816 words, and gating it would
block the build. Recorded here so the omission reads as a decision rather than
an oversight.

skill-audit's manual-fallback path listed only the line ceiling, so an agent
taking that path passed an oversized SKILL.md the hook then rejected. The word
ceiling is now named alongside it. agent-audit is deliberately unchanged: the
size hook scopes to SKILL.md only and agent-audit's validate.sh has no word
gate, so claiming it there would be false.

The Vale research doc still showed the MDX {/* vale off */} form under a
Markdown heading, contradicting CONTEXT.md and vale-run's troubleshooting
reference — that form suppresses nothing in plain .md. Fixed in both places it
appeared.

tests/run-tests.sh used mapfile (bash 4.0+) with unguarded array expansion,
though AGENTS.md tells contributors to run it and macOS ships bash 3.2. It now
collects via a while-read loop over process substitution and guards every
expansion. The newline-delimited find|sort pipeline is kept rather than -print0
with sort -z, whose BSD portability is the weaker link, and which matches
mapfile -t's previous behaviour exactly.

Refs: #85
ADR: 0013

docs/adr/0013-vale-harness-scope-and-rule-sources.md

@@ -96,10 +96,11 @@ every rule to `level: error` is what actually implements this decision.
   "cherry-picked rules" is one rule, not two.
 - A new pre-commit hook, `skill-size-check` (`scripts/skill-size-check.sh`), enforces the
   500-line/5,000-token `SKILL.md` ceiling, sibling to `skill-frontmatter`. Both halves of that
-  ceiling are blocking gates, not just the line count: `MAX_LINES=500`, and `MAX_WORDS=2900` as a
-  word-count proxy for the 5,000-token limit (calibrated from this repo's measured ~1.6-1.7 tokens
-  per word — `wc -w` is not BPE tokenization). Either one exceeded fails the hook. Both are
-  inclusive: a file at exactly 500 lines or exactly 2,900 words passes, and only one past a ceiling
+  ceiling are blocking gates, not just the line count: `MAX_LINES=500`, and `MAX_WORDS=2770` as a
+  word-count proxy for the 5,000-token limit (calibrated to the densest prose this repo measured,
+  1.81 tokens per word, so a worst-case `SKILL.md` at the ceiling still lands under 5,000 tokens —
+  `wc -w` is not BPE tokenization). Either one exceeded fails the hook. Both are
+  inclusive: a file at exactly 500 lines or exactly 2,770 words passes, and only one past a ceiling
   fails. `skill-audit/scripts/validate.sh` enforces the same pair on the same inclusive terms, so
   the audit and the commit hook cannot disagree about whether a given `SKILL.md` is over size.
 - `styles/KyberforgeTrial/` and `.vale.trial.ini` were deliberately not created — noted here so a