Add milestone planning phase between /generate-app and /add-feature

- New command /plan-milestones: generates docs/milestones/ (roadmap.md + one document per milestone with its features, acceptance criteria, and ready-to-run /add-feature commands)
- New skill milestone-planning: MVP-first vertical-slice planning rules + milestone/roadmap templates
- /add-feature: reads the feature's milestone document before planning and checks the feature off (updating milestone/roadmap status) in Step 8
- Workflow diagrams, CLAUDE.md, README, and settings.json updated for the new phase

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.claude/commands/define-design.md

@@ -25,6 +25,8 @@ claude
   ↓
 /generate-app web | flutter | winui3
   ↓
+/plan-milestones
+  ↓
 /add-feature
 ```