docs: allow UI/UX refinement before /generate-app (phase 3)

- README: add a refine loop (re-run /define-design or /update-design) between
  /define-design and /generate-app, in both Usage Workflow and the
  recommended workflow
- generate-app: note that it generates from the design as-is; refine first
- define-design: next steps now refine before generating code
- update-design: broaden to cover pre-generation refinement
- AGENTS.md: add the refine step to the initial setup flow

.opencode/command/generate-app.md

@@ -40,6 +40,7 @@ opencode
    - `docs/design/ui-blueprint.json`
    - `docs/design/platform-mapping.md`
 3. **Important**: Do **not** generate any platform-specific implementation before this command is executed. This command is the only place initial app scaffolding is created.
+4. This command generates from the design **as-is**. Refine the design first (re-run `/define-design` or use `/update-design <change>`) if anything needs to change — it is far easier to refine before code is generated than after.
 
 ## Procedure