replace send_file_dm with open_dm resolver (per review)

hive-matrix-mcp/src/protocol.rs

@@ -37,15 +37,13 @@ pub enum DaemonRequest {
         caption: Option<String>,
     },
 
-    /// Open (or reuse) a DM with `user_id` and post a local file as an
-    /// attachment. `caption`, when set, is sent as a follow-up text
-    /// message in the DM.
-    #[serde(rename = "send_file_dm")]
-    SendFileDm {
-        user_id: String,
-        path: String,
-        caption: Option<String>,
-    },
+    /// Resolve (find-or-create) the DM room with `user_id` and return
+    /// its room id, without sending anything. Lets a caller obtain the
+    /// DM room id and then use the room-based tools (`send_file`,
+    /// `send_message`, …) against it — so there is no per-tool `_dm`
+    /// variant.
+    #[serde(rename = "open_dm")]
+    OpenDm { user_id: String },
 
     /// React to a specific event with an emoji `key`. Matrix-spec
     /// `m.reaction` annotation.