New src/pm.rs module with sunbeam pm subcommand:
- Planka client: cards, boards, lists, comments, assignments
via OIDC token exchange for Planka JWT
- Gitea client: issues, comments, labels, milestones
via OAuth2 Bearer token
- Unified Ticket type with p:/g: ID prefixes
- pm list: parallel fetch from both sources, merged display
- pm show/create/comment/close/assign across both systems
- Auth via crate::auth::get_token() (Hydra OAuth2)
