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/ui/main_tui/mod.rs

@@ -489,6 +489,15 @@ mod tests {
         ) -> soulseek_rs::Result<()> {
             Ok(())
         }
+        fn watch_user(&self, _username: &str) -> soulseek_rs::Result<()> {
+            Ok(())
+        }
+        fn unwatch_user(&self, _username: &str) -> soulseek_rs::Result<()> {
+            Ok(())
+        }
+        fn watched_users(&self) -> Vec<String> {
+            Vec::new()
+        }
         fn user_info(&self, _username: &str) -> Option<soulseek_rs::UserInfo> {
             None
         }