Add changelog-synth prompt guidance

[marcus]

Summary

• add repo-aware prompt guidance for release-notes and changelog-synth
• tell changelog-synth to inspect changelog/release signals, reconcile boundary mismatches, and preserve the existing changelog format
• cover the new prompt text with focused orchestrator prompt tests

Testing

• go test ./internal/orchestrator ./internal/tasks ./cmd/nightshift/commands
• go run ./cmd/nightshift task show changelog-synth --prompt-only

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Automated by nightshift