feat: add checkpoint resume, diff, prune commands and save discoverability

Add three new CLI subcommands to improve checkpoint UX:

- `crewai checkpoint resume [id]` skips the TUI and resumes from the
  latest or specified checkpoint directly
- `crewai checkpoint diff <id1> <id2>` compares two checkpoints showing
  changes in metadata, inputs, task status, and outputs
- `crewai checkpoint prune --keep N --older-than Xd` removes old
  checkpoints from JSON dirs or SQLite databases

Also writes a resume hint to stderr after every checkpoint save so
users discover the command without needing to know it exists.