feat(#1134): add unread matrix messages as a loose end

When the local matrix daemon has rooms with unread notifications,
get_loose_ends now prepends an UnreadMatrix entry to the output so
agents see pending matrix activity alongside questions/reminders
without message content being exposed.

Changes:
- hive-sh4re: add LooseEnd::UnreadMatrix { rooms: u32 } variant
- hive-matrix-mcp: add DaemonRequest::UnreadCount and handler that
  counts joined rooms with notification_count > 0 (server-side push
  notification counts, cached by matrix-sdk)
- hive-ag3nt/mcp: inject UnreadMatrix entry on self-queries by
  connecting to /run/hive-matrix/socket (HIVE_MATRIX_SOCKET override);
  best-effort — agents without matrix configured are unaffected

hive-matrix-mcp/src/protocol.rs

@@ -81,6 +81,12 @@ pub enum DaemonRequest {
     #[serde(rename = "join_room")]
     JoinRoom { room: String },
 
+    /// Return the count of rooms with unread notifications. Used by
+    /// the harness `get_loose_ends` to surface unread matrix activity
+    /// without exposing message content.
+    #[serde(rename = "unread_count")]
+    UnreadCount,
+
     /// Liveness probe — fast "are you up?" round-trip that doesn't
     /// touch matrix-sdk. Not used by the in-tree stdio MCP bridge
     /// (which surfaces a daemon-down condition as a normal tool-call