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>

pyproject.toml

@@ -13,4 +13,9 @@ dependencies = []
 jianda-proxy = "jianda_proxy.cli:main"
 
 [tool.hatch.build.targets.wheel]
-packages = ["src/jianda_proxy"]
+packages = ["src/jianda_proxy"]
+
+[dependency-groups]
+dev = [
+    "pytest>=8.3.5",
+]