docs: add codex agent recipe example

[chubes4]

Summary

• Adds a cookbook recipe showing the minimal WP Codebox shape for a Codex-backed headless agent run.
• Documents the required runtime ingredients: Agents API, Data Machine, Data Machine Code, a php-ai-client provider-auth overlay, a Codex-enabled OpenAI provider plugin, and AI_PROVIDER_OPENAI_CODEX_* env vars.
• Clarifies the boundary: WP Codebox owns sandbox/recipe/artifact execution; fleet runners such as Homeboy own queueing, concurrency, retries, durable run records, and PR/comment workflows above it.

Tests

• node -e 'JSON.parse(require("fs").readFileSync("examples/recipes/cookbook/codex-agent-smoke.json", "utf8")); console.log("codex-agent-smoke.json valid")'

Evidence

• A local Homeboy agent-task run-plan smoke using WP Codebox, php-ai-client#238 as a runtime overlay, ai-provider-for-openai#28 as the provider plugin, and OpenCode OAuth mapped into AI_PROVIDER_OPENAI_CODEX_* succeeded.
• The Codex-backed sandbox agent responded with the Playground site title and active theme.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (openai/gpt-5.5)
• Used for: Drafted the Codex recipe example and README guidance from the successful local Homeboy/WP Codebox smoke; Chris remains responsible for review and acceptance.