Fix merged filter patterns being discarded during per-folder config init (#17)

In load_for_directory and merge_with, merged patterns were passed via
legacy exclude/include fields, but __post_init__ overwrites those from
mode-specific lists (which were empty). This caused .git/, node_modules/
and other excluded paths to be indexed for any directory with a
.cosmaconfig. Populate the correct mode-specific fields instead so
_sync_legacy_fields preserves merged patterns.

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: chmltn

packages/cosma-backend/src/cosma_backend/filter/filter_config.py

@@ -587,12 +587,21 @@ def load_for_directory(cls, directory: Path, global_config: Optional[FilterConfi
             if pattern not in merged_include:
                 merged_include.append(pattern)
 
-        merged = cls(
-            mode=global_config.mode,  # Mode always from global
-            exclude=merged_exclude,
-            include=merged_include,
-            config_path=folder_config_path,
-        )
+        # Populate mode-specific fields so __post_init__ preserves them
+        if global_config.mode == FilterMode.BLACKLIST:
+            merged = cls(
+                mode=global_config.mode,
+                blacklist_exclude=merged_exclude,
+                blacklist_include=merged_include,
+                config_path=folder_config_path,
+            )
+        else:
+            merged = cls(
+                mode=global_config.mode,
+                whitelist_exclude=merged_exclude,
+                whitelist_include=merged_include,
+                config_path=folder_config_path,
+            )
 
         logger.info("Merged filter config for directory",
                       directory=str(directory),
@@ -626,12 +635,21 @@ def merge_with(self, other: FilterConfig) -> FilterConfig:
             if pattern not in merged_include:
                 merged_include.append(pattern)
 
-        return FilterConfig(
-            mode=self.mode,
-            exclude=merged_exclude,
-            include=merged_include,
-            config_path=other.config_path or self.config_path,
-        )
+        # Populate mode-specific fields so __post_init__ preserves them
+        if self.mode == FilterMode.BLACKLIST:
+            return FilterConfig(
+                mode=self.mode,
+                blacklist_exclude=merged_exclude,
+                blacklist_include=merged_include,
+                config_path=other.config_path or self.config_path,
+            )
+        else:
+            return FilterConfig(
+                mode=self.mode,
+                whitelist_exclude=merged_exclude,
+                whitelist_include=merged_include,
+                config_path=other.config_path or self.config_path,
+            )
 
 
 class FilterConfigManager: