[0xacee]

Why this matters: Local-agent lifetime turns are counted incorrectly after partial session cleanup. If two files contain 2 and 3 turns, deleting the first makes the summary rise from 5 to 8. If cleanup and a new session offset each other, new turns can instead be missed. One aggregate file count cannot distinguish these cases.

Persist a per-file observation checkpoint and add only each retained/new file's growth. Removed history stays in the lifetime total without recounting survivors. Preserve the existing user-turn/assistant fallback rule, migrate the legacy scalar total conservatively, keep the last complete snapshot after an unreadable scan, and serialize in-process summary updates.

Validation:

• Authenticated GET /api/summary with real temporary JSONL histories, deletion, append and persisted legacy state.
• Baseline: 6 failed. Final full Token Spy suite: 28 passed, 1 skipped.
• Covers user/assistant histories, cleanup plus new files with equal or smaller aggregate size, legacy migration, and temporary read failures.
• Changed-file Ruff E/F/W and git diff --check pass.

Overlap check: all-state semantic/file search and inspection of #2798 (atomic accumulated-turn state writes) and #2691 (I/O exception narrowing). Neither changes counting after partial cleanup. #4569 addresses current-session memory and #4583 the reset target, not lifetime counting.

AI assistance: implemented and tested with Codex. Review required before merge: independent review of persisted telemetry. Existing totals are retained, not retrospectively repaired. Counts describe observed file growth; work removed between observations cannot be reconstructed. The lock coordinates threads in one process; cross-process/crash-safe persistence remains a separate concern, including #2798.

Combined validation: c08b6695ff14 on public-beta base 9fc9f61. Token Spy: 64 passed, 1 skipped, including partial-cleanup accounting, poll responsiveness, and reset-target regressions.

Merge guidance: tested Token Spy main.py order is #4561, #4569, #4583, #4634, #4640. The #4583/#4634 poller conflict must retain await asyncio.to_thread for status and reset, include_session_id=True on assessment, session_id=status["_reset_session_id"] on deletion, and cooldown updates only after action == "killed". Backlog compatibility: candidate also includes #2798; all 64 Token Spy tests pass with atomic checkpoint replacement. This PR itself still uses the existing writer.