Clarify commit normalizer task

[yimin12]

Summary

• Clarify the built-in commit-normalize task scope so agents infer the existing convention, use lightweight repo-local docs/config, and avoid unsafe history rewrites.
• Add focused registry coverage for commit-normalize metadata and required behavior text.
• Update task reference docs to match the clarified user-facing scope.

Tests

• go test ./internal/tasks
• go test ./cmd/nightshift/commands -run 'Test.*Task'
• go test ./...

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Automated by nightshift