Add studio tool for multi-agent workspace management

Studio enables running multiple g3 agents concurrently without conflicts
by using git worktrees for isolation.

Features:
- studio run --agent <name> [args...]: Create worktree, spawn g3, tail output
- studio list: Show all active sessions
- studio status <id>: Show session details and summary
- studio accept <id>: Merge session branch to main and cleanup
- studio discard <id>: Delete session without merging

Each session gets:
- Isolated worktree at .worktrees/sessions/<agent>/<session-id>
- Dedicated branch: sessions/<agent>/<session-id>
- Short UUID (8 chars) for easy reference
- Automatic --workspace and --agent flags passed to g3

Cargo.lock

@@ -3437,6 +3437,19 @@ dependencies = [
  "syn",
 ]
 
+[[package]]
+name = "studio"
+version = "0.1.0"
+dependencies = [
+ "anyhow",
+ "chrono",
+ "clap",
+ "serde",
+ "serde_json",
+ "tokio",
+ "uuid",
+]
+
 [[package]]
 name = "syn"
 version = "2.0.108"