- Add StatusHistoryEntry type to track status transitions with timestamps
- Update parser to read status_history from task frontmatter
- Update serializer to write status_history to task frontmatter
- Record status changes in backlog.ts for both task creation and updates
- Add velocity statistics panel to aggregator UI with:
- Tasks completed in last 7 days
- Average cycle time (To Do -> Done)
- Average time to start (To Do -> In Progress)
- Weekly throughput chart with 4-week visualization
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Co-Authored-By: Claude <noreply@anthropic.com>