Introduce --oss flag to use gpt-oss models (#1848)

This adds support for easily running Codex backed by a local Ollama
instance running our new open source models. See
https://github.com/openai/gpt-oss for details.

If you pass in `--oss` you'll be prompted to install/launch ollama, and
it will automatically download the 20b model and attempt to use it.

We'll likely want to expand this with some options later to make the
experience smoother for users who can't run the 20b or want to run the
120b.

Co-authored-by: Michael Bolin <mbolin@openai.com>

codex-rs/ollama/src/lib.rs

@@ -0,0 +1,52 @@
+mod client;
+mod parser;
+mod pull;
+mod url;
+
+pub use client::OllamaClient;
+pub use pull::CliProgressReporter;
+pub use pull::PullEvent;
+pub use pull::PullProgressReporter;
+pub use pull::TuiProgressReporter;
+
+/// Default OSS model to use when `--oss` is passed without an explicit `-m`.
+pub const DEFAULT_OSS_MODEL: &str = "gpt-oss:20b";
+
+/// Prepare the local OSS environment when `--oss` is selected.
+///
+/// - Ensures a local Ollama server is reachable.
+/// - Selects the final model name (CLI override or default).
+/// - Checks if the model exists locally and pulls it if missing.
+///
+/// Returns the final model name that should be used by the caller.
+pub async fn ensure_oss_ready(cli_model: Option<String>) -> std::io::Result<String> {
+    // Only download when the requested model is the default OSS model (or when -m is not provided).
+    let should_download = cli_model
+        .as_deref()
+        .map(|name| name == DEFAULT_OSS_MODEL)
+        .unwrap_or(true);
+    let model = cli_model.unwrap_or_else(|| DEFAULT_OSS_MODEL.to_string());
+
+    // Verify local Ollama is reachable.
+    let ollama_client = crate::OllamaClient::try_from_oss_provider().await?;
+
+    if should_download {
+        // If the model is not present locally, pull it.
+        match ollama_client.fetch_models().await {
+            Ok(models) => {
+                if !models.iter().any(|m| m == &model) {
+                    let mut reporter = crate::CliProgressReporter::new();
+                    ollama_client
+                        .pull_with_reporter(&model, &mut reporter)
+                        .await?;
+                }
+            }
+            Err(err) => {
+                // Not fatal; higher layers may still proceed and surface errors later.
+                tracing::warn!("Failed to query local models from Ollama: {}.", err);
+            }
+        }
+    }
+
+    Ok(model)
+}