feat(core): support custom model aliases in config.toml

This adds config.toml-defined model aliases that map to provider model slugs while applying alias-specific context settings for the active session.

- added custom_models config entries plus schema and docs coverage
- taught ModelsManager to resolve aliases to a provider-facing request_model while preserving the user-facing alias slug
- applied alias-specific context_window and model_auto_compact_token_limit overrides during model info resolution
- updated session/test plumbing and added regression coverage for alias resolution with local and remote model catalogs

Model selection and per-session context overrides already flow through ModelsManager and Config. Resolving aliases there keeps the provider slug separate from the user-facing alias while reusing the existing override plumbing.

- just write-config-schema
- just fmt
- cargo test -p codex-app-server -p codex-api -p codex-exec -p codex-mcp-server
- cargo test -p codex-tui
- cargo test -p codex-protocol -p codex-core (same two existing seatbelt failures remained in this environment: create_seatbelt_args_with_read_only_git_pointer_file and create_seatbelt_args_with_read_only_git_and_codex_subpaths)
- just fix -p codex-core -p codex-protocol -p codex-app-server -p codex-api -p codex-exec -p codex-mcp-server -p codex-tui

codex-rs/exec/src/lib.rs

@@ -479,6 +479,7 @@ async fn run_exec_session(args: ExecRunArgs) -> anyhow::Result<()> {
         auth_manager.clone(),
         SessionSource::Exec,
         config.model_catalog.clone(),
+        config.custom_models.clone(),
         CollaborationModesConfig {
             default_mode_request_user_input: config
                 .features