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, AGENTS.md, and README updated for the new phase

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

.opencode/command/generate-app.md

@@ -27,6 +27,8 @@ opencode
   ↓
 /generate-app web | flutter | winui3   ← you are here
   ↓
+/plan-milestones
+  ↓
 /add-feature
 ```
 
@@ -142,6 +144,7 @@ Generated:
 
 Next steps:
 - Run the app and verify the shell renders
+- Run /plan-milestones to break the product into milestones with a feature list each
 - Use /add-feature to implement features (they will respect the design spec)
 - Use /update-design when a feature changes the UI
 "