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, AGENTS.md, README, and docs/milestones/README.md updated

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

docs/milestones/README.md

@@ -32,6 +32,7 @@ One document per milestone. Contains the milestone's goal, scope, dependencies,
 /define-design     → UI/UX design spec
 /generate-app      → initial app shell
 /plan-milestones   → creates the files above
+/execute-milestones → implements all milestones one by one (one steering directory per milestone), updating statuses here
 /add-feature       → implements one feature, then checks it off here
 ```