chore(tasks): standardize commit guidance

[marcus]

Summary

• Centralize Nightshift commit-message guidance for planning and implementation prompts.
• Clarify commit-normalize metadata to audit recent history, align tooling/guidance, avoid history rewrites, and require trailers on new Nightshift commits.
• Add focused prompt and registry tests.

Tests

• go test ./internal/orchestrator
• go test ./internal/tasks
• go test ./...

Assumption

This standardizes future Nightshift-generated commit guidance and task metadata; it does not rewrite existing git history.

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Automated by nightshift