Add an AGENTS.md index for the loop contract

[cuddy-five]

Plan (required)

• Goal: Give agents one index of the loop, the checks, and the bans, so the next session follows issue What is the agent loop contract? #11 instead of inventing a process.
• Scope (paths/files): AGENTS.md
• Steps:
  1. Write AGENTS.md from the What is the agent loop contract? #11 contract and the checks already on main, including the Scope line.
  2. Point at the existing docs for the hook, fail-twice, scope match, Plan Gate, and MCP notes.
• Success criteria (verifiable):
  • Required checks pass
  • AGENTS.md names the three self-tests, the Scope line rule, and the seven bans
• Risks + mitigations: Docs-only. No check behavior changes.
• Rollback / escalation plan: Revert the file.

Evidence

• Workflow run(s): https://github.com/cuddy-five/agentic-cert-project/actions/runs/37085198623
• Commit SHA this run evaluated: 56d0820
• Durable proof (artifact name, repo path, or “Actions log only”): AGENTS.md
• Scan results (if applicable): n/a

Review checklist

• Plan reviewed and approved
• Required reviews satisfied
• Required checks satisfied

Refs #22. This is the index only. It does not add the ADR, the triage-label doc, or the template wording from that issue.