fix(sleep): tolerate disappearing transcript files

[kaluli123123]

Summary

• prevent Claude and Codex transcript discovery from crashing when a live session file disappears during mtime ordering
• keep harvesting the remaining files and preserve the existing digest behavior
• add regression coverage for the concurrent-removal race in both harvesters

Root cause

Both harvesters sorted discovered paths with a direct os.path.getmtime call. Live transcript stores can remove or rotate a file after directory enumeration but before sorting, allowing FileNotFoundError to abort the entire harvest.

Changed behavior

A shared _safe_mtime returns a sortable fallback for paths that disappear or become inaccessible. The later transcript read remains fail-closed, so the vanished file is skipped and other sessions continue.

Test plan

• python3 -m unittest -q tests.test_harvest_race tests.test_harvest_pi tests.test_copilot_harvest tests.test_sleep_skill_harvest tests.test_split_wiring tests.test_holdout_integrity (48 passed)
• python3 -m compileall -q skillopt_sleep tests/test_harvest_race.py (passed)
• git diff --check (passed)

The repository-required python -m pytest -q could not run because this environment has no pytest module. The OpenCode-focused unittest modules likewise require that missing dependency.

Issue

Closes #284

Incomplete gates

Full pytest suite and remote CI remain maintainer/CI gates; no merge or issue close was performed locally.