feat: enrich release-notes task prompt

[marcus]

Summary

• expand the built-in release-notes task with Nightshift-specific release drafting guidance
• add task and prompt regression tests so the richer description stays registered and reaches the planning prompt

Assumption

• this intentionally improves the built-in prompt quality rather than adding release automation, new CLI flags, or a changelog pipeline

Testing

• go test ./internal/tasks ./internal/orchestrator
• go run ./cmd/nightshift task show release-notes --project . --prompt-only

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Automated by nightshift