feat: add CLI settings export/import functionality

- Implemented exportSettings and importSettings APIs for CLI settings.
- Added hooks useExportSettings and useImportSettings for managing export/import operations in the frontend.
- Updated SettingsPage to include buttons for exporting and importing CLI settings.
- Enhanced backend to handle export and import requests, including validation and conflict resolution.
- Introduced new data structures for exported settings and import options.
- Updated localization files to support new export/import features.
- Refactored CLI tool configurations to remove hardcoded model defaults, allowing dynamic model retrieval.

ccw/src/core/services/cli-session-manager.ts

@@ -335,11 +335,15 @@ export class CliSessionManager {
     }
 
     // Merge endpoint env vars with process.env (endpoint overrides process.env)
-    const spawnEnv: Record<string, string> = {
+    const spawnEnv: Record<string, string | undefined> = {
       ...(process.env as Record<string, string>),
       ...endpointEnv,
     };
 
+    // Unset CLAUDECODE to allow nested Claude Code sessions (SDK/subagent use case)
+    // See: https://github.com/anthropics/claude-agent-sdk-python/issues/573
+    delete spawnEnv.CLAUDECODE;
+
     let pty: nodePty.IPty;
     try {
       pty = nodePty.spawn(file, args, {