fix: auto-select default model after login to prevent 'No model configured' error

When users logged in via /login but never explicitly ran /model, the agent
would throw 'No model configured' on every action. Now GSD auto-selects
a default model from available authenticated providers on startup.

Preference order: claude-sonnet-4-20250514 > any Anthropic model > first available.

Also documented /model command in README Getting Started section.

Closes #4

README.md

@@ -132,6 +132,12 @@ gsd
 
 Select from 20+ providers — Anthropic, OpenAI, Google, OpenRouter, GitHub Copilot, and more. If you have a Claude Max or Copilot subscription, the OAuth flow handles everything. Otherwise, paste your API key when prompted.
 
+GSD auto-selects a default model after login. To switch models later:
+
+```bash
+/model
+```
+
 ### Use it
 
 Open a terminal in your project and run:

src/cli.ts

@@ -17,6 +17,21 @@ await runWizardIfNeeded(authStorage)
 const modelRegistry = new ModelRegistry(authStorage)
 const settingsManager = SettingsManager.create(agentDir)
 
+// Auto-select a default model if none is configured.
+// This prevents the "No model configured" error for users who logged in
+// but never explicitly ran /model to pick one.
+if (!settingsManager.getDefaultModel()) {
+  const availableModels = modelRegistry.getAvailable()
+  if (availableModels.length > 0) {
+    // Prefer Anthropic's default (claude-sonnet-4-20250514), then any Anthropic model, then first available
+    const preferred =
+      availableModels.find((m) => m.provider === 'anthropic' && m.id === 'claude-sonnet-4-20250514') ||
+      availableModels.find((m) => m.provider === 'anthropic') ||
+      availableModels[0]
+    settingsManager.setDefaultModelAndProvider(preferred.provider, preferred.id)
+  }
+}
+
 // GSD always uses quiet startup — the gsd extension renders its own branded header
 if (!settingsManager.getQuietStartup()) {
   settingsManager.setQuietStartup(true)