Add --ignore-user-config option

[joshavant]

Summary

Add a --ignore-user-config option to codex-acp that starts the adapter without loading $CODEX_HOME/config.toml, while continuing to use CODEX_HOME for authentication and other Codex state.

This mirrors the codex exec --ignore-user-config flag added upstream in OpenAI Codex in openai/codex#18646 (be4fe9f9b21a3e3909c641c9c74e08cbaf355cc8). The option is useful for ACP clients that want to provide their own session configuration and avoid user-level hooks or MCP servers, while still using the user's existing Codex login.

The existing -c/--config overrides continue to apply when user config is ignored. The README notes that callers can pass settings such as -c cli_auth_credentials_store=auto explicitly if those settings normally come from config.toml.

Verification

• cargo fmt --all -- --check
• cargo test
• cargo clippy --all-targets --all-features -- -D warnings
• cargo run -- --help
• git diff --check