[Change] Kanban Flow Metrics + WIP/Aging Signals — Δ4

[djm81]

Why

Kanban teams won't use sprint-based commands. Today SpecFact has Policy Engine (#176) and dependency graph (#116), but no Kanban-native workflow: WIP limits, aging WIP, flow metrics (cycle time/throughput), blocked time. Without backlog flow and .specfact/kanban.yaml, Kanban teams see SpecFact as "Scrum-only."

What Changes

• Add CLI command specfact backlog flow (or backlog flow-metrics) for Kanban flow view: WIP per column, aging WIP, cycle time/throughput (when data exists), blocked time.
• Config .specfact/kanban.yaml for WIP limits, column definitions, aging thresholds; integrate with Policy Engine (feat(openspec): code review bug-finding and sidecar venv fix #176) for entry/exit policies.
• Output flow metrics (WIP, aging, cycle time, throughput) in machine-readable (JSON) and human-readable (Markdown) formats.
• Policy Engine (feat(openspec): code review bug-finding and sidecar venv fix #176) supports Kanban entry/exit policies per column when kanban config is present.
• Documentation (agile-scrum-workflows) for Kanban flow and WIP/aging.

Acceptance Criteria

• specfact backlog flow outputs WIP per column, aging, optional cycle time/throughput/blocked; JSON and Markdown.
• .specfact/kanban.yaml supports columns, WIP limits, aging thresholds; optional; Policy Engine can use for entry/exit.
• Docs updated (agile-scrum-workflows).

---

OpenSpec Change Proposal: kanban-flow-metrics