Add /execute-milestones to run milestones one by one with per-milestone steering docs

- New command /execute-milestones: executes all (or selected) milestones strictly in order; for each milestone creates a separate .steering/[date]-milestone-NN-[name]/ directory, divides its features into tasks grouped by feature, implements, validates, tests, reconciles docs, and updates milestone/roadmap statuses before starting the next
- New skill milestone-execution: per-milestone steering structure, task-division rules, gate criteria between milestones, and failure handling
- plan-milestones completion message, CLAUDE.md, README, docs/milestones/README.md, and settings.json updated

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

CLAUDE.md

@@ -102,8 +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, 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)
+- `.claude/commands/` - Slash commands (setup-project, define-design, generate-app, update-design, plan-milestones, execute-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, milestone-execution, steering)
 
 ## Development Process
 
@@ -115,7 +115,9 @@ Define "what to do this time" for a specific development task:
 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. Plan milestones with `/plan-milestones` (creates `docs/milestones/` with the features for each milestone)
-7. Implement features with `/add-feature [feature]`, milestone by milestone
+7. Implement, either way:
+   - `/execute-milestones` — execute all milestones one by one autonomously (one steering directory per milestone), or
+   - `/add-feature [feature]` — one feature at a time, milestone by milestone
 
 ### Day-to-Day Usage
 
@@ -135,9 +137,11 @@ Define "what to do this time" for a specific development task:
 > /generate-app web
 > /update-design add a profile screen
 
-# Milestone planning
+# Milestone planning and execution
 > /plan-milestones
 > /plan-milestones 3 milestones, MVP first
+> /execute-milestones
+> /execute-milestones 1
 
 # Detailed review (when a detailed report is needed)
 > /review-docs docs/product-requirements.md