docs(roadmap): reconcile changelog announcements with roadmap

Add missing changelog links to existing milestones:
- workflow-file-auto-discovery
- automatic-backend-provisioning
- path-based-component-resolution
- aws-yaml-functions
- product-roadmap

Add new milestones for shipped features:
- Auth: auth shell, SSO auto-provisioning, identity flag for describe
- DX: terminal themes, helpful errors, provenance tracking, global env,
  metadata inheritance, interactive terraform prompts
- Extensibility: !random, !env stack manifest support, circular
  dependency detection, deferred evaluation, boolean flags

Update progress percentages:
- Auth: 80% → 85%
- DX: 75% → 90%
- Extensibility: 50% → 80%

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: osterman