feat: checkpoint TUI with tree view, fork support, editable inputs/outputs

- Rewrite TUI with Tree widget showing branch/fork lineage
- Add Resume and Fork buttons in detail panel with Collapsible entities
- Show branch and parent_id in detail panel and CLI info output
- Auto-detect .checkpoints.db when default dir missing
- Append .db to location for SqliteProvider when no extension set
- Fix RuntimeState.from_checkpoint not setting provider/location
- Fork now writes initial checkpoint on new branch
- Add from_checkpoint, fork, and CLI docs to checkpointing.mdx