feat(providers): Create CliProvider abstract base class

Add reusable infrastructure for CLI-based AI providers:
- SpawnStrategy types ('wsl' | 'npx' | 'direct' | 'cmd')
- CliSpawnConfig interface for platform-specific configuration
- Common CLI path detection (PATH, common locations)
- WSL support for Windows (reuses @automaker/platform utilities)
- NPX strategy for npm-installed CLIs
- Strategy-aware subprocess spawning with JSONL streaming
- Error mapping infrastructure with recovery suggestions

Reuses existing utilities:
- spawnJSONLProcess, WSL utils from @automaker/platform
- createLogger, isAbortError from @automaker/utils

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/server/src/providers/index.ts

@@ -2,8 +2,14 @@
  * Provider exports
  */
 
-// Base provider
+// Base providers
 export { BaseProvider } from './base-provider.js';
+export {
+  CliProvider,
+  type SpawnStrategy,
+  type CliSpawnConfig,
+  type CliErrorInfo,
+} from './cli-provider.js';
 export type {
   ProviderConfig,
   ExecuteOptions,