fix(plugins): skip update for local-only plugins instead of failing (#354)

Adds a local_only flag to the starlark-apps manifest so the update
endpoint returns a skipped status rather than recording a false failure
when the plugin has no git repo and no registry entry.

Co-authored-by: Chuck <chuck@example.com>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

plugin-repos/starlark-apps/manifest.json

@@ -22,5 +22,6 @@
     "Pillow>=10.0.0",
     "PyYAML>=6.0",
     "requests>=2.31.0"
-  ]
+  ],
+  "local_only": true
 }