[Product] Add [?2004h to generate sample ~/.codex/config.toml

[github-actions]

Summary

Offer a guided command (codex init) that writes a commented ~/.codex/config.toml with common options and links, improving first‑time setup and discoverability.

Current vs Expected

• Current: Users must create config manually using docs.
• Expected: codex init prompts for key options (model, sandbox, approvals), writes a minimal template, and shows how to edit further.

Proposed Changes

• CLI subcommand init in codex (or codex config init).
• Generate a well‑commented config with examples (mcp_servers, notify, profiles).
• Respect existing file (ask to overwrite or write config.example.toml).

Acceptance Criteria

• Running codex init creates a valid config file with sensible defaults.
• Help text documents flags for non‑interactive usage.
• Docs updated (docs/config.md, getting-started).

References

• docs/config.md, codex-rs/tui/src/lib.rs (config integration)