chore: restore commit message hook

[marcus]

Summary

• add the missing commit-msg hook script to enforce normalized commit subjects for new commits
• install both pre-commit and commit-msg hooks from \✓ pre-commit hook installed (.git/hooks/pre-commit → scripts/pre-commit.sh)
  ✓ commit-msg hook installed (.git/hooks/commit-msg → scripts/commit-msg.sh)
• document the expected commit message format, allowed exceptions, and bypass guidance

Testing

• make install-hooks
• bash -n scripts/commit-msg.sh
• smoke-test valid and invalid commit message files
• go test ./...
• go build ./cmd/nightshift

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Automated by nightshift