docs: update AGENTS.md with governance directives and ADR discoverability

[scottschreckengaust]

Summary

Update AGENTS.md to surface the ADR governance framework so that a brand-new agent with ONLY AGENTS.md as a starting point can discover and follow all architectural decisions, contribution rules, and process gates.

Problem

Currently, AGENTS.md has a single routing table entry:

| Architecture decisions (ADRs) | docs/decisions/ | Run mise //docs:sync after adding or editing an ADR |

This tells agents WHERE ADRs live but not:

• That governance rules (ADR-003) exist and are binding
• That pre-start checks are required before beginning any work
• That admin approval gates block unauthorized implementation
• How to find the roadmap and verify alignment
• How to discover outstanding PRs/issues for cross-referencing

Deliverables

• Add a "Before starting work" section (or equivalent) to AGENTS.md that:
  • Directs agents to read docs/decisions/003-contribution-governance.md
  • Summarizes the critical gates (approval, priority, predecessors, cross-ref)
  • Links to the product roadmap for alignment verification
• Expand the routing table entry for docs/decisions/ with context:
  • "Binding architectural decisions — read before modifying architecture or process"
• Add a "Governance" or "Process" section that briefly explains:
  • Issue-first workflow (no PRs without issues)
  • Admin approval requirement
  • Priority evaluation (challenge lower-priority when higher exists)
  • Predecessor validation
• Reference the full ADR index (docs/decisions/README.md) for the complete framework
• Verify: an agent reading only AGENTS.md would know to check ADRs, roadmap, and open issues before starting

Blocked by

• feat(docs): ADR governance framework (ADRs 001-013) #145 (ADR governance framework) — all ADRs should be merged before this references them, ensuring stable links

Alignment

Aligns with Documentation and specifications roadmap theme and ADR-004 (tabula rasa documentation standard — AGENTS.md must be self-sufficient for agent onboarding).

Why not part of PR #143

PR #143 implements the ADR-003/004 decision records themselves. This issue updates AGENTS.md to reference those decisions. The dependency is intentional: AGENTS.md should point to merged, stable ADR content — not draft PRs. Updating AGENTS.md before the ADRs merge would create references to files that don't exist on main.

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• Approved by: (pending admin approval)
• Reviewed by: (pending)