Implement a transactional write-validate-commit-or-rollback pattern for
all skill_manage write operations (edit, patch, write_file):
- _backup_skill_file: timestamped .bak.{ts} snapshot before every write
- _validate_written_file: re-reads from disk after write to catch truncation,
encoding errors, and broken YAML frontmatter
- _revert_from_backup: restores original content (or removes the corrupted
file) on any validation failure
- _cleanup_old_backups: prunes to MAX_BACKUPS_PER_FILE (3) after success;
failed edits keep their .bak file as a debugging aid
Also fixes pre-existing issue where _patch_skill error returns lacked a
`suggestion` field expected by test_skill_manager_error_context.py tests.
Adds 21 tests in test_skill_manager_autorevert.py covering every component
and an end-to-end simulation of mid-write failure + auto-revert.
Fixes#923
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
