The git probes at AgentStart/AgentEnd blocked a tokio worker for three sequential git spawns while holding the session mutex. The three probes now run concurrently inside capture_git_context, and persist_event runs the capture via spawn_blocking outside the lock, re-taking it only to compare and append. Field semantics stay the same: every boundary re-probes git, each field stays independently optional. Sandbox numbers (4-core Linux VM): capture wall 1.29ms -> 637us (2.0x), session-lock hold per boundary 1.29ms -> 91ns.