hooks: git-commit-confirm becomes silent-unless-suspiciousHEADmain

The hook used to emit a confirmation modal on every git commit. That
produced too many benign interruptions — the modal fired even on clean,
well-formed, attribution-free commits. Now it only emits a modal when
one of these safety checks fires:

- AI-attribution patterns in the commit message (Co-Authored-By: Claude,
  robot emoji, Generated-with-AI footers, etc.) — the primary leak
- Message not parseable from command line (editor would open, which
  silently blocks Claude)
- No files staged (the commit would fail anyway)
- Git author unusable (user.name / user.email not configured)

Clean commits pass through silent. The AI-attribution scan is unchanged;
the always-on review is gone.

README updated to describe the new behavior.

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