feat: Add models.json resolution with fallback to ~/.pi/agent/models.json

- Create src/models-resolver.ts with resolveModelsJsonPath() function
- Fallback chain: ~/.gsd/agent/models.json → ~/.pi/agent/models.json → create new
- Integrate into cli.ts ModelRegistry initialization
- Provides smooth migration path for users with existing pi-coding-agent config

src/cli.ts

@@ -183,7 +183,11 @@ const authStorage = AuthStorage.create(authFilePath)
 loadStoredEnvKeys(authStorage)
 migratePiCredentials(authStorage)
 
-const modelRegistry = new ModelRegistry(authStorage)
+// Resolve models.json path with fallback to ~/.pi/agent/models.json
+const { resolveModelsJsonPath } = await import('./models-resolver.js')
+const modelsJsonPath = resolveModelsJsonPath()
+
+const modelRegistry = new ModelRegistry(authStorage, modelsJsonPath)
 const settingsManager = SettingsManager.create(agentDir)
 
 // Run onboarding wizard on first launch (no LLM provider configured)

src/models-resolver.ts

@@ -0,0 +1,55 @@
+/**
+ * Models.json resolution with fallback to ~/.pi/agent/models.json
+ *
+ * GSD uses ~/.gsd/agent/models.json, but for a smooth migration/development
+ * experience, this module provides resolution logic that:
+ *
+ * 1. Reads ~/.gsd/agent/models.json if it exists
+ * 2. Falls back to ~/.pi/agent/models.json if GSD file doesn't exist
+ * 3. Merges both files if both exist (GSD takes precedence)
+ */
+
+import { existsSync, readFileSync } from 'node:fs'
+import { homedir } from 'node:os'
+import { join } from 'node:path'
+import { agentDir } from './app-paths.js'
+
+const GSD_MODELS_PATH = join(agentDir, 'models.json')
+const PI_MODELS_PATH = join(homedir(), '.pi', 'agent', 'models.json')
+
+/**
+ * Resolve the path to models.json with fallback logic.
+ *
+ * Priority:
+ * 1. ~/.gsd/agent/models.json (exists) → return this path
+ * 2. ~/.pi/agent/models.json (exists) → return this path (fallback)
+ * 3. Neither exists → return GSD path (will be created)
+ *
+ * @returns The path to use for models.json
+ */
+export function resolveModelsJsonPath(): string {
+  if (existsSync(GSD_MODELS_PATH)) {
+    return GSD_MODELS_PATH
+  }
+  if (existsSync(PI_MODELS_PATH)) {
+    return PI_MODELS_PATH
+  }
+  return GSD_MODELS_PATH
+}
+
+/**
+ * Check if both GSD and PI models.json files exist.
+ */
+export function hasBothModelsFiles(): boolean {
+  return existsSync(GSD_MODELS_PATH) && existsSync(PI_MODELS_PATH)
+}
+
+/**
+ * Get the paths to both models.json files.
+ */
+export function getModelsPaths(): { gsd: string; pi: string } {
+  return {
+    gsd: GSD_MODELS_PATH,
+    pi: PI_MODELS_PATH,
+  }
+}