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

docs/spec/gates.md

@@ -21,31 +21,30 @@ Install hooks via `pc-run`, wiring **all three stages**. This repo's `.pre-commi
 `default_install_hook_types`, so a plain install silently skips `commit-msg` (Conventional Commits)
 and `pre-push` (everything below).
 
-The pre-push command reports **16** hooks, not 14. The extra two are pre-commit's own `meta` hooks,
+The pre-push command reports **15** hooks, not 13. The extra two are pre-commit's own `meta` hooks,
 `check-hooks-apply` and `check-useless-excludes`: they declare no `stages:`, so they run at every
 stage including this one. Both are declared in this repo's `.pre-commit-config.yaml` like everything
-else — what separates them is `repo: meta` (pre-commit's own built-ins) from `repo: local`. Fourteen
+else — what separates them is `repo: meta` (pre-commit's own built-ins) from `repo: local`. Thirteen
 is the count of hooks this repo authors itself.
 
-**The caveat: one of those 14 is a silent no-op under that invocation.**
+**The caveat: one of those 13 is a silent no-op under that invocation.**
 `check-release-needed` exits 0 immediately unless `PRE_COMMIT_REMOTE_BRANCH` equals
 `refs/heads/main`, and pre-commit exports that variable only from the real pre-push git hook during
 an actual `git push`. Running the stage by hand — or from a CI runner — therefore reports it
 `Passed` having checked nothing. That is by design for feature branches — pushing WIP must not be
 blocked on cutting a premature tag — but it means `--hook-stage pre-push --all-files` is a full
-rehearsal of 13 hooks and a skip of the fourteenth. The script's own header records the same gap for
+rehearsal of 12 hooks and a skip of the thirteenth. The script's own header records the same gap for
 a PR merged through Gitea's merge button, where no local push happens at all.
 
 ## The pre-push gate
 
-Fourteen hooks, grouped below by what they guard rather than by the order `.pre-commit-config.yaml` declares them in.
+Thirteen hooks, grouped below by what they guard rather than by the order `.pre-commit-config.yaml` declares them in.
 
 **Core checks**
 
 | Hook | Guards |
 |---|---|
 | `run-tests` | `bash tests/run-tests.sh --strict` — the whole suite, skips fatal (see [Tests](#tests)) |
-| `check-manifests` | `marketplace.json` and `plugin.json` paths resolve (needs `jq`) |
 
 **Generated-content drift gates**
 
@@ -92,8 +91,8 @@ and `check-plugin-content-sync` (via `scripts/sync-plugin-content.sh`, which wra
 first and third are bare `apm …` entries and the second is a `bash -c` loop calling `apm` once per
 package, so without the CLI the push dies with an unhelpful "command not found". Install with
 `apm-install`, or `curl -sSL https://aka.ms/apm-unix | sh`; verify with `apm --version`. `jq` is
-needed by `scripts/check-manifests.sh` and `scripts/sync-plugin-content.sh` — those at least fail
-loudly (`Error: jq is required but not installed`).
+needed by `scripts/sync-plugin-content.sh` — it at least fails loudly (`Error: jq is required but
+not installed`).
 
 ## Skill and agent context gates (ADR-0020)