feat: add --check-stale flag for CI gating (fixes #2)

[Fuhua-GIF]

Summary

Adds --check-stale flag to build.py for CI pipeline gating, as requested in #2.

Changes

• Updated build.py:

  • Added --check-stale CLI flag: exits 1 if older (non-current-commit) diagnostic artifacts exist
  • Added --max-stale-bytes option (default: 0 = any stale is an error)
  • Added find_stale_artifacts() function: scans diagnostic/ for artifacts from other commits
  • Added check_stale_artifacts() function: compares artifact commit IDs to current HEAD
  • Added 5 self-tests (--self-test) covering empty dir, current artifacts, stale detection, byte thresholds, and chunked artifacts
  • The flag is read-only and does not delete any artifacts

• Updated docs/OPERATIONS.md:

  • Added 'Build System' section documenting --check-stale
  • Documented options table, CI pipeline YAML example, and self-test instructions

Usage

# CI gate: fail if stale diagnostics exist
python3 build.py --check-stale

# Allow up to 1KB of stale artifacts
python3 build.py --check-stale --max-stale-bytes 1024

# Run self-tests
python3 build.py --self-test

Testing

All 5 self-tests pass:

1. Empty diagnostic dir is clean
2. Current commit artifacts are not stale
3. Different commit artifact is stale
4. max_stale_bytes threshold works
5. Chunked stale artifacts are detected

Diagnostic Artifacts

Diagnostic JSON metadata: build-ec966bc5.json

Note: .logd file could not be generated on Windows (encryptly binary not available for this platform). JSON metadata is included.

Checklist

• --check-stale flag added to build.py
• --max-stale-bytes option works
• 5 self-tests added and passing
• Documentation updated in docs/OPERATIONS.md
• Flag is read-only (does not delete artifacts)
• Changes are scoped to the build system

Fixes #2