docs: update README for CLI tool usage, remove old script

- Rewrite README.md with CLI usage instructions
- Add __main__.py for `python -m jianda_proxy` support
- Clean up unused imports in config.py and test_config.py
- Add pytest dev dependency to pyproject.toml
- Remove lfs-proxy.py (replaced by modular CLI tool)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tests/test_config.py

@@ -1,7 +1,4 @@
-import os
-import tempfile
 import pytest
-from configparser import ConfigParser
 from jianda_proxy.config import (
     get_config_dir,
     get_config_path,
@@ -11,7 +8,6 @@ from jianda_proxy.config import (
     set_config_value,
     get_config_value,
     list_config,
-    DEFAULTS,
 )
 
 FIXTURES = {