docs(briefings): add diagram responsibilities to Qatux and project docs

- CLAUDE.md: add docs/diagrams/ to project structure, add diagram
  file conventions for D-record changes
- qatux.md: add priority 5 for diagram creation/updates
- workshop-start SKILL.md: add diagram step in wrap-up sequence

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/briefings/qatux.md

@@ -35,13 +35,15 @@ None assigned directly.
 2. Create round documents directly at `docs/discussions/round-NN-topic.md` (D-022 workflow)
 3. Update `docs/discussions/README.md` with new round entries after formal closure
 4. Update relevant agent briefing files with new decision references
-5. Re-index changed documents in Qdrant after updates
-6. Verify briefing freshness against decision domain files
+5. **Create and update diagrams** whenever D-records are added or modified — use the `/d2-diagram` skill to generate d2 source + PNG. New decisions get a diagram if they describe systems, flows, or relationships. Existing diagrams in `docs/diagrams/{category}/` must be updated when their source decisions change. Categories: architecture, data-flow, entity, state, ui.
+6. Re-index changed documents in Qdrant after updates
+7. Verify briefing freshness against decision domain files
 
 ## Key Documents
 - `decisions/` — domain-split decision files (see `decisions/README.md` for index)
 - `docs/discussions/` — archived rounds (including active rounds per D-022)
 - `docs/briefings/` — agent briefing files (you maintain these)
+- `docs/diagrams/` — decision diagrams (architecture, data-flow, entity, state, ui) — `.d2` source + `.png` renders
 - `docs/workshops/` — workshop briefs, outputs, and summaries
 - `TEAM.md` — team roster