S06 — Coverage, cases, sessions

Plan

Operator surfaces for gap coverage, integration case list/run, and session dual-wiki browse. Thin executor writes sessions/<id>/.

Do

1. Add GET /api/cases and GET /api/sessions (plus POST /api/cases/{id}/run) in studio-server/studio_server.py.
2. Build Coverage, Cases, and Sessions rail pages consuming registry JSON.
3. Smoke-run one existing case (e.g. agency) from Studio; confirm session appears.

Check

scripts/fi-studio-sprint-pdca/check-phase-gate.sh S06

Act

Fix API routes or session writer; re-run S06 gate until green.