Remove dead code and fix stale documentation
- Remove unused get_entries_for_range() from cluster.py
- Remove deprecated cluster_segments_multi alias from cluster.py
- Remove unused cancel_task() from supervisor.py
- Fix type hints: any -> Any in cluster_segments()
- Update planner.md: journal://insight -> journal://agents
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>