[codex] add checkpoint replay guide

[salim4n]

Closes #57

What changed

• Added a guide for turning a trained checkpoint into a persisted inference run and replay.
• Covers checkpoint listing, checkpoint detail, inference, replay, Studio artifact refresh and current supported compatibility pairs.
• Documents common checkpoint/replay failure modes.
• Linked the guide from the main README.

Validation

• Verified the documented Target2D demo/checkpoints/checkpoint/infer/replay commands against a local project.
• bun run ci