End an active checkpoint. Rewind context to it, replacing intermediate exploration with your report.

Call immediately after `checkpoint`-started investigative work.

Requirements:
- `report` is REQUIRED and MUST be concise, factual, and actionable.
- Include key findings, decisions, and any unresolved risks.
- AVOID raw scratch logs unless essential.
- You MUST call this before yielding if a checkpoint is active.

Behavior:
- If no checkpoint is active, this tool errors.
- On success, the session rewinds and keeps your report as retained context.
