fix(loaders): render clean signatures, drop unresolvable aliases from models

- Stringify Object.signature() instead of str()-ing the bound method,
  which produced "<bound method Class.signature of ...>" reprs
- Skip alias members that cannot resolve (stdlib/third-party imports)
  while preserving resolvable package re-exports; return None for empty
  signatures (classes without __init__ args)
- Add MCP renderer regression tests for signature cleanliness, alias
  filtering, and package re-export preservation

docs/mcp/modules/docforge.cli.main.json

@@ -8,14 +8,14 @@
         "name": "cli",
         "kind": "attribute",
         "path": "docforge.cli.main.cli",
-        "signature": "<bound method Alias.signature of Alias('cli', 'docforge.cli.commands.cli')>",
+        "signature": null,
         "docstring": null
       },
       "main": {
         "name": "main",
         "kind": "function",
         "path": "docforge.cli.main.main",
-        "signature": "<bound method Function.signature of Function('main', 13, 21)>",
+        "signature": "main() -> None",
         "docstring": "Run the doc-forge command-line interface.\n\nThis function initializes and executes the Click CLI application.\nIt is used as the console entry point when invoking `doc-forge`\nfrom the command line."
       }
     }