fix: key stall termination on read watermark, not prompt lock (0.14.3)

[william0wang]

Problem

Since #85 (0.14.2), silently-running turns — most visibly sub-agent tasks — were killed after 120s of stream silence. Zed.log captured two live hits:

10:08:56 [stall] no-progress deadline reached; prompt lock=released
10:08:56 [stop] v4/command stop sent for sess_31dbe177

Root cause

#85's deadline check probed the prompt lock via session/goal show and killed the turn whenever the probe did not report the 1308 lock-busy error. Raw-backend probes (.zcode/scratch/probe-goal-lock.mjs) against the Aug-28 app-server proved the prompt lock is not a liveness signal at all:

• session/goal show succeeds mid-turn (11/11 probes across a 185s live turn, projection running the whole time) — it never reports 1308;
• a probe session/send is ACCEPTED while the turn runs (queued as steer input) — the lock is only held during turn finalisation;
• the read projection's watermark (contextUsed/turnCount) keeps advancing the whole time a sub-agent works behind a silent stream (msgs 2→20, contextUsed 0→147k over 5 minutes with zero session/events).

So "lock released" is the normal state of a healthy mid-turn backend, and killing on it murdered live sub-agent turns after 2 quiet minutes.

Fix

Replace the lock probe with a watermark heartbeat taken from the existing 15s stall-reconcile session/read (zero extra RPCs):

• watermark advancing → backend demonstrably alive → defer the terminal decision indefinitely;
• watermark frozen < 10 min (STALE_FREEZE_MS) → still defer (long CoT / quiet tools legitimately freeze it for 60s+);
• watermark frozen ≥ 10 min → stale projection confirmed: fetch the reply and end gently (end_turn, stallRecovered), bounded stop only when nothing was ever delivered.

This keeps #85's convergence goal (a projection stuck at running no longer hangs forever) while never killing a live turn. probePromptLock is deleted; tests assert the goal channel is never consulted again.

Verification

• tests/stale-running-recovery.test.ts rewritten: watermark-advancing turn survives 120s and 10 min of silence and ends normally on turn.completed; frozen watermark converges to max_turn_requests after the 10-min budget (with stop); frozen + delivered output ends gently without stop.
• Full suite 838/838 green; typecheck/lint/build/smoke clean; smoke reports 0.14.3.
• Docs: docs/ARCHITECTURE.md turn-state diagram updated; new AGENTS.md gotcha "Prompt lock ≠ turn liveness" with the probe evidence.