fix: scope changelog synth to documented branch

[marcus]

Summary

• treat Nightshift's configured branch as the branch being documented for changelog synthesis, not the comparison base
• update changelog-synth task text, prompt guidance, tests, and docs to derive commits from the documented branch back to its inferred base
• keep generic tasks free of changelog-specific guidance and verify the full Go test suite still passes

Testing

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