fix(session): fence turn-failure writes under the run lease; detect paused turns without the .paused marker (Eve2-F2, Eve2-F3)

[LinuxDevil]

What

Fixes two HIGH durability findings from the eve2 runtime-tools audit.

Eve2-F2 — recordFailure() poisoned a foreign checkpoint

AgentSession.recordFailure() did a raw load + save merging lastError { retryable: false } / attempts + 1 onto whatever checkpoint occupied <id>.turn-<n>, bypassing the run-lease fencing that saveStepCheckpoint/RunLease.save use. A run that stalled past its lease, then failed permanently, stamped its error on the checkpoint of the process that had taken the turn over — bricking that turn forever with LOUSHO_SESSION_TURN_FAILED.

Fix: the session now takes the turn's run lease itself before the run starts (holdRunLease), hands it to the run via the internal HELD_RUN_LEASE option (so the executor saves under it instead of taking its own), keeps it until record() is done, and writes the failure through RunLease.save — a fenced save that throws LOUSHO_SESSION_BUSY, writing nothing, once the lease is lost. Stores without lease support behave exactly as before.

Eve2-F3 — .paused marker gap

The <id>.paused pointer is written by markPaused() after the executor already saved the awaiting-approval checkpoint — two non-atomic durable writes. assertNotPaused consulted only the marker, so a agent.send(msg, { sessionId }) in the gap (or after a crash between the writes, permanently) ran a full turn beside the paused one.

Fix (option a, defense-in-depth): pausedSessionTurn now falls back to scanning <id>.turn-* checkpoints via CheckpointStore.list({ status: 'awaiting-approval' }) when the pointer is absent (or stale), returning the newest paused turn. Stores without list() keep the pointer-only behavior; a list() that is present but unavailable (KV binding without list) degrades gracefully.

Tests

• src/session/sessionLease.test.ts: stalled session turn (file + SQLite stores) fails permanently after losing its lease — verifies the new holder's checkpoint keeps its own attempts: 1, lastError: { retryable: true } and stays resumable.
• src/session/pausedSessionLock.test.ts: .paused marker deleted after a pause (the crash window) — agent.send(msg, { sessionId }), session.pending() and session.send() all still refuse with SessionAwaitingApprovalError.

Gates

• npx tsc --noEmit -p tsconfig.json clean (tests tsconfig too)
• npm run lint clean
• npx vitest run src/session src/execution — 85 files / 1188 tests pass

No public API changes (HELD_RUN_LEASE / RunLeaseOptions already existed internally; no signature changes).