v0.12.1 — audit hardening sweep

Clears the remaining low-priority hardening notes from the v0.11.0 audit (docs/11 section C — now fully consumed). Regression suite 203 → 205.

Fixed

• Stop hooks now verify the writeSync return value when appending an event line; a short write (e.g. ENOSPC) is truncated back to the pre-write size and reported, so a fragment can never corrupt the next event line.
• latestAgentDoneEvent no longer goes silently blind past 1 MB of events: it reads a bounded 64 KB tail (dropping the first partial line), keeping agent_event_seen / last_turn_id metadata live on long sessions — and reading less than before for mid-size files.
• Non-agent sessions skip the agent-metadata directory probe on every pty_read via a 2-second absence-only negative cache (read-suffix path only; invalidated on openAgent / closeSession / killAll).