fix(core): normalize path separators to forward slashes on Windows

path.relative() produces backslashes on Windows but manifest JSON
should always use forward slashes for cross-platform portability.

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

Author: killagu

packages/core/src/loader/manifest.ts

@@ -224,10 +224,8 @@ export class ManifestStore {
   // --- Path Utilities ---
 
   #toRelative(absPath: string): string {
-    if (path.isAbsolute(absPath)) {
-      return path.relative(this.baseDir, absPath);
-    }
-    return absPath;
+    const rel = path.isAbsolute(absPath) ? path.relative(this.baseDir, absPath) : absPath;
+    return rel.replaceAll(path.sep, '/');
   }
 
   #toAbsolute(relPath: string): string {