fix: remove agents field from plugin.json manifest (#1459)

The Claude Code plugin validator rejects the "agents" field entirely.
Remove it from the manifest, schema, and tests. Update schema notes
to document this as a known constraint alongside the hooks field.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

schemas/plugin.schema.json

@@ -31,10 +31,6 @@
       "type": "array",
       "items": { "type": "string" }
     },
-    "agents": {
-      "type": "array",
-      "items": { "type": "string" }
-    },
     "features": {
       "type": "object",
       "properties": {