fix: make AssetManager registry path relative to storage_path by default

This is a robust fix for test registry isolation that addresses the root cause:
when AssetManager is created with only storage_path, the registry now defaults
to storage_path.parent/asset_registry.json instead of cwd/asset_registry.json.

Benefits:
- Tests using temp directories automatically get isolated registries
- No need to manually fix every test file
- Consistent behavior: registry stays with the asset storage
- Explicit registry_path still works for custom configurations

This makes the AssetManager behavior more intuitive and prevents test
artifacts from contaminating the production asset registry.

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

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

markitect/assets/manager.py

@@ -64,8 +64,10 @@ class AssetManager:
                 assets_config.get('storage_path', DEFAULT_ASSETS_DIR)
             ).resolve()
 
+            # Default registry path should be relative to storage_path, not cwd
+            default_registry_path = self.storage_path.parent / DEFAULT_REGISTRY_FILENAME
             self.registry_path = Path(
-                assets_config.get('registry_path', DEFAULT_REGISTRY_FILENAME)
+                assets_config.get('registry_path', default_registry_path)
             ).resolve()
 
             self.database_path = Path(