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/cli.rs

@@ -207,6 +207,10 @@ pub enum Commands {
     /// What the server knows about another user
     User(UserArgs),
 
+    /// Keep a daemon updated about users you care about
+    #[command(subcommand)]
+    Watch(WatchCommand),
+
     /// Confirm the credentials and connection work
     Whoami,
 
@@ -571,6 +575,26 @@ pub struct WishRunArgs {
     pub sort: SortKey,
 }
 
+/// Watching only lasts as long as the session doing it, so these are aimed at
+/// a running daemon: it stays connected and keeps receiving the status pushes.
+#[derive(Subcommand, Debug)]
+pub enum WatchCommand {
+    /// Start watching a user
+    Add {
+        /// The user to watch
+        user: String,
+    },
+
+    /// Stop watching a user
+    Remove {
+        /// The user to stop watching
+        user: String,
+    },
+
+    /// List the users being watched
+    List,
+}
+
 #[derive(Subcommand, Debug)]
 pub enum RoomCommand {
     /// List public rooms and their user counts