fix: resolve restrict-template-expressions lint error in claude-process.ts

Wrap `subtype` with String() to convert from `unknown` to `string` before
using in template literal. This was the sole lint error causing CI failure
in workflow run #912.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Claude (Webhook)

server/claude-process.ts

@@ -308,7 +308,7 @@ export class ClaudeProcess extends EventEmitter<ClaudeProcessEvents> implements
 
     // Log non-streaming event types for diagnostics
     if (event.type !== 'stream_event') {
-      const subtype = 'subtype' in event ? (event as Record<string, unknown>).subtype : '-'
+      const subtype = 'subtype' in event ? String((event as Record<string, unknown>).subtype) : '-'
       console.log(`[event] type=${event.type} subtype=${subtype || '-'}`)
     }
     // Log all event types we DON'T handle to catch unknown protocol messages