docs(adr): correct the records the branch left describing deleted things

Seven ADRs described code that no longer exists or behaviour the gates do
not have. Where the wrong text came from main it carries a dated
correction; where this branch introduced it, it is fixed in place, because
main never published it and there is no record to preserve.

Fixed in place, branch-introduced:

- ADR-0021's 2026-09-14 correction asserted apm audit --ci "was never a
  drift gate at all". It is one: it replays the install and diffs. The
  claim contradicted this branch's own AGENTS.md and gates.md.
- ADR-0015 said unconditionally that no pre-push hook needs the network.
  The guarantee holds only once apm install has populated apm_modules/.
- ADR-0014's 2026-09-16 correction said restoring .pre-commit-hooks.yaml
  would ship a hook that fails for every consumer, because their checkout
  has no lib-boundary-resolver.sh. pre-commit clones the whole hook repo
  and skill-size-check.sh resolves the library from BASH_SOURCE, so the
  hook would work.
- ADR-0019's "twelve hooks pass under unshare -rn" matched neither HEAD
  (8) nor main (14), and stated the offline guarantee unconditionally.

Corrected, inherited from main:

- ADR-0022 and ADR-0013 named skill-frontmatter's pre-commit hook as the
  enforcer of mandatory metadata.version. That hook was deleted on this
  branch; the check lives in skill-size-check.sh.
- ADR-0022 enumerated the tip rule's carve-outs as a closed list and
  described a single merge-base. The gate also exempts a tree-identical
  skill and intersects every base from merge-base --all, and emits a third
  failure form. 8cfd54f said the documented behaviour did not change; it
  did. The gate is correct and is unchanged -- the record was not.
- ADR-0020's Decision still routed description overflow to README.md, its
  ADR-0025 amendment pointed the mirrored constants at validate.sh, which
  holds none, and its Enforcement table still named the two deleted
  validate.sh paths.
- ADR-0015's Status claimed every plugin's plugin.json is pack output;
  none exist.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NwD8Egs5r4ndqeFLmhusX2

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

@@ -26,7 +26,11 @@ stay out of this Vale-based harness because this repo already has dedicated tool
 `skill-frontmatter` (required frontmatter fields), `validate-marketplace`
 (`claude plugin validate --strict`, schema), and `gitleaks`/`detect-private-key` (secrets).
 (ADR-0024 removed the companion `validate-plugins` gate along with the per-plugin manifests it
-checked; the argument here is unaffected.)
+checked, and the `skill-frontmatter` hook has since been removed as well — its required-field
+checks were folded into `skill-size-check`, and the enforcer is now
+`scripts/skill-size-check.sh:324-335` under the `skill-size-check` hook at
+`.pre-commit-config.yaml:237`. The argument here is unaffected either way: a dedicated
+non-Vale tool still owns required frontmatter fields.)
 Duplicating those concerns as Vale rules would fight tools that already own them better.
 
 **Governance docs are excluded as a rule source.** `docs/research/governance_principles/CONTROLS.md`