C1: Add _sessions_lock to protect all compound mutations and iterations
on the global _sessions dict across 5+ concurrent execution contexts
(main dispatcher, pool workers, daemon threads, notification poller,
atexit handler).
C2: Add _prompt_lock to protect _pending/_pending_prompt_payloads/_answers
dicts from races between _block() (agent callback thread) and
_respond() (pool worker). Lock scope is kept tight — _block() only
holds the lock during registration/cleanup, releasing it before
_emit() and ev.wait() to avoid blocking other prompts for 300s.
All 187 existing TUI tests pass with no regressions.
asill-livestream
5bcb63e400