Add three new flags for filtering convoys by epic relationship:
- --orphans: show only convoys without a parent epic
- --epic <id>: show only convoys under a specific epic
- --by-epic: group convoys by parent epic
These support the Goals Layer feature (Phase 3) for hierarchical
focus management.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>