docs(matrix-tools): add missing open_dm to matrix tool list

open_dm(user_id) resolves (find-or-create) the DM room and returns
its room id without sending anything. It is the counterpart to
send_dm for cases where you need a room id to pass to a room-based
tool such as send_file or send_message.

The tool was present in hive-matrix-mcp (mcp.rs, handlers.rs,
protocol.rs) and the terminal-rendering icon table but was missing
from the tools/matrix.md reference doc.

docs/tools/matrix.md

@@ -17,6 +17,11 @@ room you haven't read yet.
   accepts `!id:server` or `#alias:server`, daemon resolves either
 - `send_dm(user_id, body)` — open (or reuse) the DM room with
   `user_id` and post `body` to it
+- `open_dm(user_id)` — resolve (find-or-create) the DM room with
+  `user_id` and return its room id **without sending anything**.
+  Use the returned id with room-based tools (`send_message`,
+  `send_file`, …) to deliver into a DM when `send_dm`'s body
+  parameter is inconvenient (e.g. for file attachments)
 - `send_reply(room, event_id, body)` — threaded reply to a specific
   event
 - `send_reaction(room, event_id, key)` — react to a message with an