chore: delete the check-manifests pre-commit hook

Six pre-push hooks were validating overlapping sets of the same
manifests. check-manifests (marketplace.json/plugin.json path checks)
is redundant with validate-plugins (claude plugin validate) and
apm-pack-check-clean, which already cover the same ground.

Deletes the check-manifests hook entry, scripts/check-manifests.sh
(282 lines), and tests/test-check-manifests.sh (771 lines).
scripts/lib/marketplace-plugins.sh is kept — it is still sourced by
sync-plugin-content.sh. Updates the now-stale check-manifests.sh
mentions and hook counts in README.md and docs/spec/gates.md.

The apm-audit-ci and apm-marketplace-check hooks named in the same
finding are left untouched — the audit flags them as needing a
separate decision.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YR2CjVumUbEGWcMikcoXBD

SIMPLIFICATION-AUDIT.md

@@ -59,7 +59,8 @@ Five suites account for 215 s of 276 s. Three of those five (sync-plugin-content
 
 This is the area you named as hardest to understand and slowest. Root cause: most pre-push hooks exist to keep two copies of something in sync, or to re-validate what another hook already validates.
 
-1. **Six hooks validate overlapping sets of the same manifests.** `check-manifests`, `validate-plugins`, `validate-marketplace`, `apm-pack-check-clean`, `apm-marketplace-check`, `apm-audit-ci`. Keep the two `claude plugin validate` hooks plus `apm-pack-check-clean`. Delete `check-manifests` (282 lines + 771 test lines; its `lib/marketplace-plugins.sh` stays because `sync-plugin-content.sh` sources it). `apm-audit-ci` spends 12 s confirming that manifests `apm pack` already parses do parse; drop or keep on that basis. Move the network-dependent `apm-marketplace-check` to a release checklist. Effort S.
+1. **Six hooks validate overlapping sets of the same manifests.** `check-manifests`, `validate-plugins`, `validate-marketplace`, `apm-pack-check-clean`, `apm-marketplace-check`, `apm-audit-ci`. Keep the two `claude plugin validate` hooks plus `apm-pack-check-clean`. ~~Delete `check-manifests` (282 lines + 771 test lines; its `lib/marketplace-plugins.sh` stays because `sync-plugin-content.sh` sources it).~~ `apm-audit-ci` spends 12 s confirming that manifests `apm pack` already parses do parse; drop or keep on that basis. Move the network-dependent `apm-marketplace-check` to a release checklist. Effort S.
+   > **Done (2026-09-12):** see commit `9051d14` on `docs/simplification-audit`. Deleted the `check-manifests` pre-commit hook entry, `scripts/check-manifests.sh` (282 lines), and `tests/test-check-manifests.sh` (771 lines); kept `scripts/lib/marketplace-plugins.sh`, still sourced by `sync-plugin-content.sh`. Updated the now-stale `check-manifests.sh` mentions in `README.md` and `docs/spec/gates.md` (hook table row and hook counts). The `apm-audit-ci` and `apm-marketplace-check` decisions in this finding remain open — out of scope for this change.
 
 2. **Four "keep two copies in sync" gates: 1,100 script lines + 1,600 test lines.** Each one is a symptom of duplication that could be removed instead of guarded:
    - `check-vale-style-sync`: 413 lines + 798 test lines guarding a byte-identical 526-line `vale-wrap.sh` and style directory copied between skill-audit and agent-audit. About 350 of its lines run Vale glob probes against the hook file patterns. Disappears if the two audit skills merge (finding 14); the probes belong in `test-vale-wrap.sh`.