feat(codex): AGENTS.md surfacing beyond status line #36

Post-MVP follow-up to #13. MVP loads workspace-root AGENTS.md and surfaces it in the status line. This issue extends to whatever else codex actually consumes from AGENTS.md that affects observable behavior.

Scope

Research what real codex does with AGENTS.md (the research-codex-coverage skill plus the codex source is the authority). Replicate the observable surface — anything that affects argv response, hook payloads, slash-command dispatch, etc.

Non-observable surfaces (e.g., codex feeding AGENTS.md content into the model prompt) are not relevant for testagent.

Non-goals

• Anything that requires a model to act on AGENTS.md content.
• Modifying the MVP status-line read.

Sequencing

After #13 MVP ships.