chore: remove stale check-manifests references left by e647f14

e647f14 deleted scripts/check-manifests.sh but missed three live files
that still named it: the apm-marketplace-check hook description in
.pre-commit-config.yaml, and comments in sync-plugin-content.sh and
lib/marketplace-plugins.sh explaining design decisions by pointing at
a script that no longer exists.

Also deletes list_marketplace_remote_plugin_names from
marketplace-plugins.sh — its only caller was check-manifests.sh, so
it's been dead code since that commit.

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

.pre-commit-config.yaml

@@ -104,7 +104,7 @@ repos:
 
       - id: apm-marketplace-check
         name: apm marketplace check
-        description: Validate every marketplace.packages[] entry resolves, including network reachability of remote refs -- catches stale/unreachable remote package references that check-manifests.sh deliberately skips (local-source checks only)
+        description: Validate every marketplace.packages[] entry resolves, including network reachability of remote refs -- the only hook that checks remote package references rather than local-source paths
         entry: apm marketplace check
         language: system
         stages: [pre-push]