claude-plugins: add matrix-agent specialized plugin

claude-plugins/.claude-plugin/marketplace.json

@@ -11,6 +11,12 @@
       "description": "hyperhive's base plugin - skills every agent needs, regardless of role. Currently: state-hygiene (read-before-write discipline and dated-archive convention for durable agent notes/state files). Add a new skill here (not a new plugin) if it applies to all agents; give it its own specialized plugin if it doesn't.",
       "source": "./plugins/base",
       "category": "productivity"
+    },
+    {
+      "name": "matrix-agent",
+      "description": "hyperhive's Matrix-presence plugin - conventions for agents with a Matrix chat room/DM presence. Specialized, not every agent has one.",
+      "source": "./plugins/matrix-agent",
+      "category": "productivity"
     }
   ]
 }