Update quickstart and examples to include Codex CLI agent option

[sergeyklay]

Update the quickstart guide and WORKFLOW.md examples on the docs site to show Codex CLI as an alternative agent option alongside Claude Code and Copilot CLI.

Changes:

• Add a Codex tab/section to the quickstart showing agent.kind: codex configuration
• Add a Codex WORKFLOW.md example file to the examples section
• Update the "Supported agents" table/list on the overview page to include Codex

Keep additions minimal. The goal is discoverability, not duplication of the reference page (#400).

Depends on #238 (Codex adapter implementation), #400 (Codex reference documentation), #458 (Codex tutorial).

Verify: quickstart mentions Codex as an option. Codex WORKFLOW.md example is published.