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/settings.json

@@ -11,6 +11,7 @@
       "Skill(design-tokens)",
       "Skill(platform-ui-generation)",
       "Skill(milestone-planning)",
+      "Skill(milestone-execution)",
       "Skill(steering)"
     ],
     "deny": [],