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-sh4re/src/lib.rs

@@ -290,6 +290,14 @@ pub enum LooseEnd {
         due_at: i64,
         age_seconds: u64,
     },
+    /// Unread matrix notifications in one or more rooms. Not cancellable —
+    /// use `mark_read` via the matrix MCP to clear. Injected by the
+    /// in-container harness (not hive-c0re) because the matrix daemon
+    /// runs inside the agent container.
+    UnreadMatrix {
+        /// Number of rooms with at least one unread notification.
+        rooms: u32,
+    },
 }
 
 /// Kind discriminator for `CancelLooseEnd`. Per-kind store +