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

.pre-commit-config.yaml

@@ -75,15 +75,6 @@ repos:
         pass_filenames: false
         always_run: true
 
-      - id: check-manifests
-        name: Check plugin manifests
-        description: Validate marketplace.json and plugin.json paths
-        entry: bash scripts/check-manifests.sh
-        language: system
-        stages: [pre-push]
-        pass_filenames: false
-        always_run: true
-
       - id: check-plugin-content-sync
         name: Check plugin content sync
         description: Verify each plugin's flat skills/agents/commands/hooks/hooks.json mirror is in sync with .apm/ -- Claude Code has no .apm/ awareness so this compiled mirror must stay current (see issue #90)