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.md

@@ -86,6 +86,11 @@ Define "what to build" and "how to build it" for the entire application:
 - **screens/\*.svg** - Visual wireframes (references only)
 - Created by `/define-design`, updated by `/update-design`, consumed by `/generate-app` and `/add-feature`
 
+#### Milestone Documents (`docs/milestones/`)
+- **roadmap.md** - Milestone overview: table, ordering rationale, current milestone, icebox
+- **milestone-[NN]-[name].md** - One per milestone: goal, scope, and the features it contains (each sized for one `/add-feature` run)
+- Created by `/plan-milestones`; `/add-feature` checks features off as it completes them
+
 ### Work-Unit Documents (`.steering/`)
 
 Define "what to do this time" for a specific development task:
@@ -97,8 +102,8 @@ Define "what to do this time" for a specific development task:
 ### Claude Code Configuration (`.claude/`)
 
 - `.claude/agents/` - Subagent definitions (doc-reviewer, implementation-validator)
-- `.claude/commands/` - Slash commands (setup-project, define-design, generate-app, update-design, add-feature, review-docs)
-- `.claude/skills/` - Task-specific skills (prd-writing, functional-design, architecture-design, repository-structure, development-guidelines, glossary-creation, ui-design, design-tokens, platform-ui-generation, steering)
+- `.claude/commands/` - Slash commands (setup-project, define-design, generate-app, update-design, plan-milestones, add-feature, review-docs)
+- `.claude/skills/` - Task-specific skills (prd-writing, functional-design, architecture-design, repository-structure, development-guidelines, glossary-creation, ui-design, design-tokens, platform-ui-generation, milestone-planning, steering)
 
 ## Development Process
 
@@ -109,7 +114,8 @@ Define "what to do this time" for a specific development task:
 3. Define the UI/UX design with `/define-design` (creates `docs/design/`)
 4. Review and refine the UI/UX with `/update-design` (or re-run `/define-design`) until approved — before generating code
 5. Generate the initial app with `/generate-app web | flutter | winui3`
-6. Implement features with `/add-feature [feature]`
+6. Plan milestones with `/plan-milestones` (creates `docs/milestones/` with the features for each milestone)
+7. Implement features with `/add-feature [feature]`, milestone by milestone
 
 ### Day-to-Day Usage
 
@@ -129,6 +135,10 @@ Define "what to do this time" for a specific development task:
 > /generate-app web
 > /update-design add a profile screen
 
+# Milestone planning
+> /plan-milestones
+> /plan-milestones 3 milestones, MVP first
+
 # Detailed review (when a detailed report is needed)
 > /review-docs docs/product-requirements.md
 ```