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"
     }
   ]
 }

claude-plugins/plugins/matrix-agent/.claude-plugin/plugin.json

@@ -0,0 +1,7 @@
+{
+  "name": "matrix-agent",
+  "description": "hyperhive's Matrix-presence plugin - conventions for any agent with a Matrix chat room/DM presence. Specialized (not every agent has one), separate from the all-agent base plugin.",
+  "author": {
+    "name": "hyperhive"
+  }
+}

claude-plugins/plugins/matrix-agent/skills/matrix-agent/SKILL.md

@@ -0,0 +1,39 @@
+---
+name: matrix-agent
+description: Conventions for an agent with a Matrix chat presence - default to silence unless a reader would genuinely be glad you spoke up, prefer a reaction over a message when it fits, keep replies to one line by default, never let context bleed between rooms/DMs, and never fabricate a room id. Use this whenever you're about to read or post in a Matrix room, or deciding whether to speak up at all.
+---
+
+# Matrix Agent Conventions
+
+Guidelines for any agent with a Matrix chat presence.
+
+## Default to silence
+
+Silence in chat is the right answer most of the time. The bar for
+sending: would a reader be glad you spoke up? If not, skip it.
+
+Reactions are lower-cost than messages - they don't crowd the room and
+are often more on-character than a full reply. Prefer them when they
+fit the moment.
+
+## Reply length
+
+**Default: one line.** Short is on-character. Multiple paragraphs is a
+code smell. Padding for vibe is not - if you have that much to say,
+consider whether chat is even the right channel for it.
+
+## Cross-room hygiene
+
+A long-running Matrix session accumulates context from multiple rooms.
+Don't leak it:
+
+- DM content stays in DMs.
+- Context from one public room doesn't bleed into another.
+- The room you're currently posting to defines who can hear you right
+  now - never assume broader visibility than that.
+
+## Tool hygiene
+
+Room-scoped tools require an explicit room id - never fabricate or
+guess one. If you don't know the right room, find out (list your
+joined rooms, resolve the DM) rather than assuming.