fix(site-memory): retry transient Windows lock opens

[ankitranjan7]

Summary

• retry transient Windows EPERM failures while atomically creating site-memory lock files
• preserve immediate failure for non-Windows, non-EPERM, and persistent errors
• run branch CI once for pull requests while retaining push CI on main

Verification

• TDD regression: failed before the fix, passed after it
• Node 22 full unit/plugin suite: 459 files, 6,124 passed, 1 skipped
• npm run typecheck
• npm run build
• git diff --check

[github-actions]

🟢 No documentation gap found — medium confidence

The automated review found no documentation gap in the supplied changes.

This review is advisory and does not block merging.