fix: make changelog-synth branch-aware

[marcus]

Summary

• make changelog-synth prompts use Nightshift's configured base branch when available instead of hardcoding main
• fall back to an inferred <base> comparison with explicit assumptions when no branch metadata is available
• update task/docs copy and add regression coverage for the new branch-aware behavior

Testing

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