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/daemon/proto/mod.rs

@@ -106,6 +106,9 @@ wire_vocabulary! {
         BrowseUser => "browse.user",
         UserRequest => "user.request",
         UserInfoOf => "user.info",
+        UserWatch => "user.watch",
+        UserUnwatch => "user.unwatch",
+        UserWatched => "user.watched",
         SharesStatusOf => "shares.status",
         SharesSet => "shares.set",
         SharesReindex => "shares.reindex",