#217 Replay: Add --resume-from CLI support for partial replay

## Problem No official API to resume from a specific step. Must replay from beginning. ## Missing Features - No `--resume-from <step_name>` CLI flag - No way to skip N steps and start from step N+1 - No state validation before partial resume ## Impact - Slow debugging for long workflows - Must wait through all steps to debug later ones - Workaround: Manually delete checkpoints (risky) ## Solution 1. Add `hwe replay <run_id> --resume-from <step>` flag 2. Validate checkpoint state before resume 3. Skip steps up to resume point 4. Document behavior clearly ## Effort: Medium