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

README.md

@@ -243,6 +243,13 @@ Creates the design specification under `docs/design/` (design brief, design toke
 platform mapping, and SVG wireframes). You'll be asked about target platform, visual style, main
 screens, navigation, branding, and accessibility; if you don't specify, sensible defaults are proposed.
 
+Review and refine the UI/UX **before** generating the app — it is far easier to iterate on the design
+than on generated code:
+
+- Re-run `/define-design` to change the overall direction.
+- Run `/update-design <change>` for targeted refinements (e.g. `/update-design make the home screen denser`).
+- Iterate until the design — especially `docs/design/ui-blueprint.json` — is approved.
+
 **The design source of truth is `docs/design/ui-blueprint.json`.** SVG files are visual references only.
 
 ### 3. Generate the app
@@ -300,7 +307,9 @@ Recommended workflow:
 
 1. Run `/setup-project`
 2. Run `/define-design`
-3. Review `docs/design/`
+3. Review `docs/design/` and refine the UI/UX until approved:
+   - Re-run `/define-design` to change direction, or
+   - Run `/update-design <change>` for targeted refinements
 4. Run `/generate-app web`, `/generate-app flutter`, or `/generate-app winui3`
 5. Use `/add-feature` to add features
 6. Use `/update-design` whenever a feature changes the UI