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/settings.json

@@ -10,6 +10,7 @@
       "Skill(ui-design)",
       "Skill(design-tokens)",
       "Skill(platform-ui-generation)",
+      "Skill(milestone-planning)",
       "Skill(steering)"
     ],
     "deny": [],