feat: add optional thread metadata repair

[vibeforge1111]

Summary

• report oversized thread title and first-message preview metadata
• add opt-in --repair-thread-metadata-bloat mode with targeted restore artifacts
• document the three-mode workflow and backup privacy caveat
• extend smoke tests to prove normal apply does not trim metadata

Test plan

• python -m py_compile scripts\keep_codex_fast.py tests\smoke_test.py
• python tests\smoke_test.py
• local structure validation matching CI
• tracked-file safety scan matching CI