skillopt-sleep harvest crashes when a transcript disappears during discovery

[kaluli123123]

Summary

The Claude and Codex transcript harvesters can abort an entire sleep cycle when a live session file is removed or rotated between directory discovery and mtime sorting.

Reproduction

On main at 79124b37e9a6371e13b753f8bcd7adb1e493ade1, make os.path.getmtime raise FileNotFoundError for one collected transcript path, or delete a collected file before sorting:

# harvest.py and harvest_codex.py currently sort with:
paths.sort(key=lambda p: os.path.getmtime(p), reverse=True)

The exception escapes before any digest is returned. This is reachable during normal live-session cleanup/rotation, not only through malformed input.

Expected behavior

A vanished transcript should be skipped and harvesting should continue with the remaining files, matching the existing defensive behavior in the Pi harvester.

Actual behavior

FileNotFoundError escapes from the sort key and aborts the harvest, so skillopt-sleep run cannot process the night's remaining sessions.

Impact

A concurrent session cleanup can turn an otherwise recoverable read race into a failed nightly run.

Environment

SkillOpt-Sleep on main at 79124b37e9a6371e13b753f8bcd7adb1e493ade1; reproducible with Python 3.10+ and a mocked disappearing transcript path.

[kaluli123123]

I am working on a focused fix for this race. I will add regression coverage for vanished transcript paths, make mtime ordering fail closed for only the missing file, and run the focused harvest tests plus the available repository checks.

[kaluli123123]

Implemented in PR #285: https://github.com/microsoft/SkillOpt/pull/285\n\nRoot cause: Claude and Codex harvesters called os.path.getmtime directly while sorting discovered transcript paths, so a live session rotation could raise FileNotFoundError and abort the whole harvest. The fix adds a shared fail-closed mtime helper; the vanished file is skipped while remaining sessions continue.\n\nFocused validation: 48 unittest cases passed across the new race regression, Claude/Codex-adjacent harvesters, split wiring, and holdout integrity; compileall and git diff --check passed. The required pytest suite remains unrun because this environment has no pytest module.\n\nThe PR is open and ready for maintainer review; no merge or Issue close was performed.