fix: guard yaml.safe_load, flock unlock, TOCTOU races, and atomic writes

[vanthinh6886]

Summary

Fixes 4 classes of bugs across 9 files:

1. yaml.safe_load() crash on empty config (HIGH)

File: trajectory_compressor.py
yaml.safe_load() returns None for empty files. The subsequent if 'tokenizer' in data raises TypeError. Fix: add or {} fallback.

2. Unguarded fcntl.flock(LOCK_UN) in finally blocks (MEDIUM x6)

Files: cron/scheduler.py, hermes_cli/auth.py, agent/shell_hooks.py, tools/skill_usage.py, tools/environments/file_sync.py, tools/memory_tool.py

If flock(LOCK_UN) raises OSError, fd.close() is skipped, leaking the fd and holding the lock forever. The msvcrt branches already had try/except; the fcntl branches did not. Fix: wrap in try/except (OSError, IOError): pass.

3. TOCTOU race in copilot_acp_client.py (MEDIUM)

path.exists() followed by path.read_text() with no exception handling. Fix: use try/except FileNotFoundError.

4. Non-atomic cache write in sticker_cache.py (MEDIUM)

Path.write_text() leaves truncated JSON on crash. Fix: tempfile + fsync + os.replace() (atomic).

Test Plan

• All 9 modified files pass python3 -m py_compile
• No new lint errors introduced
• CI passes

[BoardJames-Bot]

BoardJames triage: current branch-local blocker is check-attribution, not the code change itself. CI reports the new author email vanthinh6886@gmail.com (vanthinh6886) is missing from scripts/release.py AUTHOR_MAP; GitHub email search returned no public login for that email, but the PR author/login is vanthinh6886, so the narrow fix is to add:

"vanthinh6886@gmail.com": "vanthinh6886",

to AUTHOR_MAP and rerun checks. All other completed checks are green so far (ruff + ty, ruff enforcement, Windows footguns, nix ubuntu/macos, supply-chain, common-ancestor, e2e, build-amd64). Tests / test and build-arm64 were still running when I checked, so I’m not attributing those pending states to the branch yet.

[outsourc-e]

Reviewed locally in a clean worktree. I spot-checked the touched paths and compiled all 9 modified files. The changes are narrowly defensive: safe_load fallback on empty config, fail-safe unlock cleanup, TOCTOU hardening, and atomic sticker-cache writes. I did not find a new security concern in the implementation.

[teknium1]

Merged via PR #28593 (cherry-picked onto current main with your authorship preserved via rebase-merge — commit 62573f4). Thanks for the contribution!