fix: resolve md-ingest Path object conversion error

Fix critical runtime error where md-ingest command failed with
'str' object has no attribute 'exists'. The DocumentManager.ingest_file()
method expects a Path object but was receiving a string from the CLI.

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

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

markitect/plugins/builtin/markdown_commands.py

@@ -1533,7 +1533,7 @@ def md_ingest_command(ctx, file_path):
         doc_manager = DocumentManager(config.get('db_manager'))
 
         # Process the file
-        result = doc_manager.ingest_file(file_path)
+        result = doc_manager.ingest_file(Path(file_path))
 
         if config.get('verbose', False):
             click.echo(f"Processing results:")