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>

.opencode/command/plan-milestones.md

@@ -136,7 +136,8 @@ Milestone documents created:
 
 Next steps:
 - Review docs/milestones/ and adjust priorities if needed
-- Start milestone 1: run /add-feature [first feature of milestone 1]
-- /add-feature marks each feature complete in its milestone document as it finishes
+- Run /execute-milestones to implement all milestones one by one autonomously (a separate steering directory is created per milestone)
+- Or go feature by feature: run /add-feature [first feature of milestone 1]
+- Either way, each feature is marked complete in its milestone document as it finishes
 "
 ```