feat: matrix MCP invite_user tool

Adds an invite_user tool to the per-agent matrix MCP so an agent can invite
another user to a room it's already in (e.g. pull a peer into an existing
chat). Mirrors the existing join_room/send_dm wiring:

- protocol.rs: DaemonRequest::InviteUser { room, user_id }
- handlers.rs: invite_user — parse user_id, resolve room, room.invite_user_by_id
- socket.rs: dispatch arm
- bin/mcp.rs: invite_user tool + InviteUserArgs

room accepts an id or alias; the caller must hold a power level high enough
to invite (the homeserver error is surfaced verbatim otherwise). The invitee
then accepts via join_room. allowedTools is already '*' for the matrix server,
so the tool is exposed without a nix change.

hive-matrix-mcp/src/bin/mcp.rs

@@ -127,6 +127,15 @@ struct JoinRoomArgs {
     room: String,
 }
 
+#[derive(Debug, Deserialize, JsonSchema)]
+struct InviteUserArgs {
+    /// Matrix room id (`!abc:server`) or canonical alias (`#name:server`)
+    /// to invite the user into. You must already be a member.
+    room: String,
+    /// Matrix user id of the invitee (`@user:server`).
+    user_id: String,
+}
+
 struct MatrixBridge {
     #[allow(dead_code)]
     tool_router: rmcp::handler::server::router::tool::ToolRouter<Self>,
@@ -237,6 +246,22 @@ impl MatrixBridge {
         render(round_trip(DaemonRequest::JoinRoom { room: args.room }).await)
     }
 
+    #[tool(
+        description = "Invite a user to a matrix room you're already in. `room` is a \
+            room id (!abc:server) or alias (#name:server); `user_id` is the invitee \
+            (@user:server). You must have a high enough power level in the room to \
+            invite. The invitee then sees a pending invite they accept with `join_room`."
+    )]
+    async fn invite_user(&self, Parameters(args): Parameters<InviteUserArgs>) -> String {
+        render(
+            round_trip(DaemonRequest::InviteUser {
+                room: args.room,
+                user_id: args.user_id,
+            })
+            .await,
+        )
+    }
+
     #[tool(
         description = "List the members of a matrix room (joined-state only). \
             Each row carries the user id and resolved display name."