feat(maestro): carry a blocked-work indicator on Session (#83)

Session.blocked_work is Option<InTheWay>, refreshed by the daemon's
watcher; None for worktree-less sessions and until first probed.
Defaults on the wire so older payloads stay forward-compatible.

crates/maestro-testing/src/lib.rs

@@ -43,6 +43,7 @@ pub fn session(id: Uuid, cwd: &str, command: &str) -> Session {
         activity: Default::default(),
         last_activity_at: Utc::now(),
         current_cwd: None,
+        blocked_work: None,
     }
 }