fix: use nullish coalescing in loadDotEnv for Sonar S6606

Co-authored-by: Cursor <cursoragent@cursor.com>

src/env.ts

@@ -12,7 +12,7 @@ export function loadDotEnv(cwd = process.cwd()): void {
       if (i <= 0) continue;
       const key = trimmed.slice(0, i).trim();
       const value = trimmed.slice(i + 1).trim();
-      if (process.env[key] === undefined) process.env[key] = value;
+      process.env[key] ??= value;
     }
   } catch {
     // optional .env