Docs: improve config reference and add config.toml.example

[fitchmultz]

• Summary:
  • Overhauled docs/config.md to fix defaults, valid values, and clarify usage:
  • Default model is "gpt-5"; default approval_policy is "on-request".
  • Documented built-in providers: "openai" (Responses API; endpoint differs for ChatGPT sign-in vs API key) and "oss" (Chat Completions).
  • Correct per-provider defaults: request_max_retries=4, stream_max_retries=5, stream_idle_timeout_ms=300000.
  • Corrected shell environment filtering defaults to KEY/TOKEN only.
  • Improved the config reference table (clear allowed values and defaults; linked sandbox/auth docs).
  • Expanded examples: OpenAI Chat Completions, Azure with api-version, local OSS, workspace-write sandbox, MCP, profiles, shell env policy.
  • Replaced “unless-allow-listed” with the accepted value “untrusted”.
• Added a comprehensive starter file docs/config.toml.example for users to copy to ~/.codex/config.toml, with safe commented sections (providers, sandbox, shell env, profiles, MCP, notify).
• Updated README.md and codex-rs/README.md to link to docs/config.toml.example.
• Added a tip in docs/advanced.md (MCP section) linking to docs/config.toml.example.
• Rationale:
  • The previous docs lacked a clear, copyable example and had conflicting defaults. This consolidates accurate, code-validated settings in a single place and improves onboarding.
• Validation:
  • Defaults and accepted values cross-checked against Rust sources:
  • Defaults: core/src/config.rs, core/src/model_provider_info.rs, protocol/config_types.rs.
  • Responses endpoints and OPENAI_BASE_URL behavior: core/src/model_provider_info.rs and chatgpt client usage.
• Example TOML validates structurally; sections are commented to prevent accidental side effects (e.g., MCP, notify).
• Links verified: docs/config.md#mcp_servers, docs/sandbox.md, docs/authentication.md, docs/config.toml.example.
• Impact:
  • Documentation only; no behavior changes.

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[fitchmultz]

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