fix(kyberforge): keep .github/plugin/marketplace.json synced

apm.yml only declares claude and codex marketplace output profiles --
codex writes a differently-shaped file to .agents/plugins/marketplace.json,
so nothing regenerates the legacy Copilot CLI path at
.github/plugin/marketplace.json. It was hand-synced once during the #90
conversion and had drifted (missing the category field added when codex
output was enabled) with no gate to catch it.

scripts/sync-marketplace-mirror.sh keeps it byte-identical to the compiled
.claude-plugin/marketplace.json, wired as a pre-push check so it can't go
stale silently again.

Refs: #90

.pre-commit-config.yaml

@@ -71,6 +71,15 @@ repos:
         pass_filenames: false
         always_run: true
 
+      - id: check-marketplace-mirror-sync
+        name: Check marketplace mirror sync
+        description: Verify .github/plugin/marketplace.json (Copilot CLI's legacy manifest path) is byte-identical to .claude-plugin/marketplace.json -- apm has no output profile for this path, so it must be kept in sync explicitly (see issue #90)
+        entry: bash scripts/sync-marketplace-mirror.sh --check
+        language: system
+        stages: [pre-push]
+        pass_filenames: false
+        always_run: true
+
       - 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)