Improve secrets push protection bypass gates

[DENGXUELIN]

Summary

• add push protection bypass governance checks for bypass events, delegated bypass policy, reviewer independence, fix-later tracking, rotation/revocation proof, and allowlist deltas
• extend the report output with a push protection bypass governance table
• add vulnerable and benign fixtures covering unreviewed fix-later bypasses versus reviewed synthetic test-token bypasses

Validation

• git diff --check origin/main...HEAD
• Markdown fence balance check
• Added-line ASCII check
• Content marker check for push protection, bypass event, delegated bypass, fix-later, rotation, revocation, allowlist, and reviewer independence
• git merge-tree --write-tree origin/main HEAD

Closes #1673

Bounty request: Improver Moderate / USD 100 if accepted.