The staleness warning in sprint-status always fired after 7 days because
it checked the 'generated' timestamp, which is only set once during
sprint-planning. Other workflows (dev-story, create-story, code-review,
retrospective) update statuses but never touched 'generated'.
This adds a 'last_updated' field that is:
- Set initially alongside 'generated' during sprint-planning
- Bumped to current date by every workflow that modifies sprint-status.yaml
- Used by the staleness check (with fallback to 'generated' for backward
compatibility)
Fixesbmad-code-org/BMAD-METHOD#1820
Co-authored-by: Oz <oz-agent@warp.dev>