feat(daemon): expose watching over the RPC surface and the CLI

The watch subscription was reachable only from the library. It is most
useful against a daemon -- that is the session that stays connected long
enough to receive the server's status pushes -- so it needs the wire.

- proto: user.watch / user.unwatch / user.watched, with a Watched response
- SessionApi gains watch_user / unwatch_user / watched_users, so a local and
  a remote session are still interchangeable above the seam
- CLI: `watch add`, `watch remove`, `watch list`, documented in SKILL.md
  including the caveat that a one-shot run's watch ends with the run
- docs/openrpc.json regenerated from the wire types

soulseek-rs/src/output.rs

@@ -383,6 +383,25 @@ impl Record for RoomMemberRecord {
     }
 }
 
+/// One user this session is watching, with whatever the server has already
+/// said about them. The status is `None` until the watch reply lands.
+#[derive(Debug, Clone, PartialEq, Eq, Serialize)]
+pub struct WatchedUserRecord {
+    pub user: String,
+    /// `online`, `away`, or `offline`.
+    pub status: Option<String>,
+}
+
+impl Record for WatchedUserRecord {
+    fn text(&self) -> String {
+        format!(
+            "{}\t{}",
+            sanitize(&self.user),
+            self.status.clone().unwrap_or_else(|| "-".to_string())
+        )
+    }
+}
+
 /// One configured share folder.
 #[derive(Debug, Clone, PartialEq, Eq, Serialize)]
 pub struct ShareRecord {