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

.gitignore

@@ -31,3 +31,6 @@ logs/
 requirements.md
 todo.g3.md
 tmp/
+
+# Studio worktrees
+.worktrees/