fix(pptx): add retry, timeout, and lock cleanup to thumbnail soffice conversion

[voidborne-d]

Summary

Fixes #886 — thumbnail.py's convert_to_images() calls soffice --headless --convert-to pdf once with no retry, no timeout, no --norestore flag, and no stderr capture. In containerized environments (Docker, sandboxed VMs), LibreOffice fails intermittently due to stale lock files, concurrent processes, or profile corruption. The only error message was a bare "PDF conversion failed" with zero diagnostics.

Root Cause

The xlsx/scripts/recalc.py in this same repo already uses --norestore and timeouts for its soffice calls, but pptx/scripts/thumbnail.py was written without these safeguards. The discrepancy means thumbnail generation is fragile in exactly the environments where the PPTX skill is most commonly used (containerized Claude Code sessions).

Changes

skills/pptx/scripts/thumbnail.py

1. Retry loop (3 attempts): on soffice failure or timeout, clean up stale lock files and retry before giving up
2. --norestore flag: prevents LibreOffice from trying to recover a previous session (parity with recalc.py)
3. 120s timeout: prevents soffice from hanging indefinitely on corrupted profiles or deadlocked processes; also added to the pdftoppm call
4. stderr in error messages: RuntimeError messages now include the actual soffice/pdftoppm stderr output for debugging
5. _cleanup_soffice_lock() helper: removes stale .~lock.* files from both the temp dir and the default LibreOffice user profile between retries

skills/pptx/scripts/tests/test_thumbnail_soffice_retry.py (new)

18 regression tests organized in 3 test classes:

• TestSourceAudit (7 tests): AST-level verification that --norestore is present, timeout= is set on both subprocess calls, retry/timeout constants are defined, lock cleanup function exists, no bare error messages without stderr detail remain, and pdftoppm has timeout
• TestConvertToImagesRetry (8 tests): functional tests via stubbed module loading — success on first/second attempt, all retries exhausted with stderr detail, timeout triggers retry, all timeouts exhausted, pdftoppm timeout/error with stderr, --norestore flag presence in actual command list
• TestCleanupLockFunction (3 tests): lock file removal, no-op on empty dirs, non-lock file preservation

All tests use lightweight stubs (no PIL/defusedxml/soffice required), suitable for CI.

Testing

python3 -m pytest skills/pptx/scripts/tests/test_thumbnail_soffice_retry.py -v
# 18 passed