Move retro.rs and retro_agent.rs into a new fabro-retro crate to reduce
the size of fabro-workflows and clarify domain boundaries.
Key design changes:
- Add CompletedStage struct as a flat DTO that decouples retro derivation
from Checkpoint/Outcome types in the workflow engine
- derive_retro now takes Vec<CompletedStage> (owned) instead of &Checkpoint
- run_retro_agent takes an event_callback closure instead of EventEmitter,
pushing event filtering to the caller
- Shared build_completed_stages() in fabro-workflows::lib converts
Checkpoint → Vec<CompletedStage> for both run.rs and server.rs callers
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
