feat: refine commit-normalize guidance

[marcus]

Summary

• refine the built-in commit-normalize task instructions so they infer existing commit style and standardize future commit guidance
• add prompt regression coverage for the new commit-normalize safety guardrails
• update the public task reference to match the forward-looking, no-history-rewrite behavior

Testing

• go test ./internal/tasks ./internal/orchestrator ./cmd/nightshift/commands
• ./nightshift task show commit-normalize --prompt-only

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Automated by nightshift