Slash commands run on the event-loop thread, outside the per-turn
contextvars.copy_context() that pins the session cwd for the agent call.
/compress reaches agent._build_system_prompt(), whose "Current working
directory" line comes from resolve_agent_cwd() — so an unpinned handler
rebuilt the prompt against the Hermes install tree and PERSISTED it as
the session's cached prompt, re-poisoning every later turn even though
the turn itself is now pinned.
Pin inside a fresh context copy so the write cannot leak into other
concurrent ACP sessions on the shared loop and needs no teardown.
