AGENTS.md instruction-file renderer for Codex/Gemini

[Ryan-Atkinson87]

Render the canonical instruction source into AGENTS.md — the instruction file Codex and Gemini consume — so the engine can produce the provider-appropriate file at run start for non-Claude providers, alongside the existing Claude CLAUDE.md + skill renderer (#22).

Spec §3.2 (provider matrix — Codex/Gemini → AGENTS.md), §3.4 (instruction-file generation), §17.7 (rendering).

Acceptance criteria

• The provider-format renderer (Provider-format renderer (compose personas to CLAUDE.md + skill files) #22) gains an AGENTS.md output path, selected when the resolved provider is Codex or Gemini.
• Composes the same persona set (Persona composition (type x speciality with stage-skill filtering) #20) and execution profile (execution-profile.yaml schema and fail-fast loader #21) into a single AGENTS.md — one canonical source, provider-appropriate file, no Claude-specific skill-file layout.
• No content drift between CLAUDE.md and AGENTS.md: both derive from the one canonical set (§3.4).
• Rendering is deterministic and re-runnable (idempotent overwrite) per §17.7.
• executor: engine rules are not rendered into the AI-facing AGENTS.md (§17.3).
• Unit tests cover Codex-selected and Gemini-selected rendering each producing AGENTS.md.

Depends on: #22