feat: Implement Accessibility Planner agent + reviewer + 5 framework skills

[WilliamBerryiii]

Summary

Implement the Accessibility Planner: a planning agent, accessibility reviewer, 5 framework standards-as-skills, and supporting state schema + instructions, following the implementation plan at .copilot-tracking/plans/2026-05-09/accessibility-planner-plan.instructions.md.

Architecture follows Decision 5c-final (standards-as-skills + assessor-subagent) and applies reduction levers L1–L7. Research is captured in .copilot-tracking/research/2026-05-08/accessibility-planner-research.md.

Scope

• New planner agent: .github/agents/accessibility/accessibility-planner.agent.md
• New reviewer agent: .github/agents/accessibility/accessibility-reviewer.agent.md
• New subagents (research, assessor) under .github/agents/accessibility/subagents/
• Five framework skills (standards-as-skills):
  • wcag-22
  • aria-apg
  • coga
  • section-508
  • en-301-549
• State schema: accessibility-state.schema.json + schema-mapping.json registration
• Phase/coaching/handoff instructions under .github/instructions/accessibility/
• License Posture instructions (.github/instructions/shared/license-posture.instructions.md)
• Collection updates: hve-core-all, project-planning (+ regenerate plugins)
• Docs updates

Phases

1. Branch setup (feat/accessibility-planner)
2. Foundations (state schema, schema-mapping, License Posture instructions)
3. Standards-as-skills (5 framework skills, ~42 reference files; parallelizable)
4. Subagents + reviewer
5. Planner agent + instructions + L3/L5 shared extractions
6. Integration + docs (collections, npm run plugin:generate, npm run extension:prepare)
7. Validation (lint:all, validate:skills, lint:frontmatter, test:ps, lint:py, plugin:validate, lint:collections-metadata)
8. STOP — hard stop for manual review before commit/PR
9. Commit + PR

Acceptance

• All 9 phases complete with passing validation
• Plan checklist fully checked in .copilot-tracking/plans/2026-05-09/accessibility-planner-plan.instructions.md
• Changes log finalized in .copilot-tracking/changes/2026-05-09/accessibility-planner-changes.md
• PR opened from feat/accessibility-planner

References

• Plan: .copilot-tracking/plans/2026-05-09/accessibility-planner-plan.instructions.md
• Details: .copilot-tracking/details/2026-05-09/accessibility-planner-details.md
• Research: .copilot-tracking/research/2026-05-08/accessibility-planner-research.md