fix: resolve critical worker crashes on startup (v8.0.2 regression)

Fixes #417, #418, #421, #422, #425

1. Handle Chroma sync errors gracefully in DatabaseManager to prevent unhandled promise rejections.

2. Safely handle missing 'claude' executable in SDKAgent with try-catch block around auto-detection.

src/services/worker/DatabaseManager.ts

@@ -31,7 +31,9 @@ export class DatabaseManager {
     this.chromaSync = new ChromaSync('claude-mem');
 
     // Start background backfill (fire-and-forget)
-    this.chromaSync.ensureBackfilled();
+    this.chromaSync.ensureBackfilled().catch(error => {
+      logger.error('DB', 'Chroma backfill failed (non-fatal)', {}, error);
+    });
 
     logger.info('DB', 'Database initialized');
   }

src/services/worker/SDKAgent.ts

@@ -435,15 +435,30 @@ export class SDKAgent {
    */
   private findClaudeExecutable(): string {
     const settings = SettingsDefaultsManager.loadFromFile(USER_SETTINGS_PATH);
-    const claudePath = settings.CLAUDE_CODE_PATH ||
-      execSync(process.platform === 'win32' ? 'where claude' : 'which claude', { encoding: 'utf8', windowsHide: true })
-        .trim().split('\n')[0].trim();
-
-    if (!claudePath) {
-      throw new Error('Claude executable not found in PATH');
+    
+    // 1. Check configured path
+    if (settings.CLAUDE_CODE_PATH) {
+      // Lazy load fs to keep startup fast
+      const { existsSync } = require('fs');
+      if (!existsSync(settings.CLAUDE_CODE_PATH)) {
+        throw new Error(`CLAUDE_CODE_PATH is set to "${settings.CLAUDE_CODE_PATH}" but the file does not exist.`);
+      }
+      return settings.CLAUDE_CODE_PATH;
     }
 
-    return claudePath;
+    // 2. Try auto-detection
+    try {
+      const claudePath = execSync(
+        process.platform === 'win32' ? 'where claude' : 'which claude', 
+        { encoding: 'utf8', windowsHide: true, stdio: ['ignore', 'pipe', 'ignore'] }
+      ).trim().split('\n')[0].trim();
+      
+      if (claudePath) return claudePath;
+    } catch (error) {
+      logger.debug('SDK', 'Claude executable auto-detection failed', error);
+    }
+
+    throw new Error('Claude executable not found. Please either:\n1. Add "claude" to your system PATH, or\n2. Set CLAUDE_CODE_PATH in ~/.claude-mem/settings.json');
   }
 
   /**