feat: generate the model catalog from live API data

Add a hidden --generate-models flag that rebuilds models.yaml from
models.dev and OpenRouter merged with the hand-owned quirks overlay,
regenerate the committed catalog with it (393 -> 1090 models), and
retire the Python updater. GitHub becomes fully quirks-owned since
upstream only tracks Copilot ids, legacy ids upstream dropped are
preserved as hand-owned quirks entries, and non-chat model families
(image/tts/audio/realtime) are excluded from generation.

src/client/model.rs

@@ -358,7 +358,7 @@ impl ModelData {
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct ProviderModels {
     pub provider: String,
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "Option::is_none")]
     pub oauth: Option<OAuthConfig>,
     pub models: Vec<ModelData>,
 }