feat: add plugin store index

- Add store.json for plugin listing
- Auto-generate store.json in CI workflow

.github/workflows/sign.yml

@@ -31,10 +31,13 @@ jobs:
           bash scripts/sign-all.sh /tmp/private.key
           rm -f /tmp/private.key
 
-      - name: Commit signatures
+      - name: Generate store.json
+        run: bash scripts/generate-store.sh > store.json
+
+      - name: Commit changes
         run: |
           git config user.name "GitHub Actions"
           git config user.email "actions@github.com"
-          git add -A "plugins/**/*.sig"
-          git diff --staged --quiet || git commit -m "chore: update plugin signatures"
+          git add -A "plugins/**/*.sig" store.json
+          git diff --staged --quiet || git commit -m "chore: update signatures and store"
           git push