feat: auto-detect provider from ~/.claude/settings.json

Reads ANTHROPIC_BASE_URL and ANTHROPIC_API_KEY from Claude Code's
settings.json and registers them as a 'claude' provider. Automatically
sets it as the default provider when no custom default is configured.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/model/app_config.rs

@@ -101,6 +101,44 @@ impl AppConfig {
         for (name, provider) in defaults.providers {
             cfg.providers.entry(name).or_insert(provider);
         }
+        // Auto-detect provider from ~/.claude/settings.json
+        if let Some(claude_provider) = detect_claude_settings_provider() {
+            cfg.providers.entry("claude".to_string()).or_insert(claude_provider);
+            if cfg.default_provider == defaults.default_provider {
+                cfg.default_provider = "claude".to_string();
+            }
+        }
         cfg
     }
 }
+
+/// Configuration structure inside `~/.claude/settings.json`.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+struct ClaudeEnv {
+    #[serde(alias = "ANTHROPIC_BASE_URL")]
+    anthropic_base_url: Option<String>,
+    #[serde(alias = "ANTHROPIC_API_KEY")]
+    anthropic_api_key: Option<String>,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+struct ClaudeSettings {
+    env: Option<ClaudeEnv>,
+}
+
+/// Read `~/.claude/settings.json` and return a `ProviderConfig` if the file
+/// contains `ANTHROPIC_BASE_URL` and `ANTHROPIC_API_KEY` in its `env` block.
+fn detect_claude_settings_provider() -> Option<ProviderConfig> {
+    let path = dirs::home_dir()?.join(".claude").join("settings.json");
+    let content = std::fs::read_to_string(&path).ok()?;
+    let settings: ClaudeSettings = serde_json::from_str(&content).ok()?;
+    let env = settings.env?;
+    let base_url = env.anthropic_base_url?;
+    let _api_key = env.anthropic_api_key?; // presence check — stored as env var, not in config.
+    Some(ProviderConfig {
+        api_base: base_url,
+        api_key_env: Some("ANTHROPIC_API_KEY".to_string()),
+        default_model: None,
+        default_api_key: None,
+    })
+}