fix: remove clear=True from test_env to preserve PATH on Windows

The test_env fixture was using mocker.patch.dict(os.environ, ..., clear=True)
which cleared ALL environment variables including PATH. This broke git
operations on Windows where git.exe cannot be found without PATH.

Since tests are already protected by comprehensive mocking (Coder.create,
InputOutput, --exit flags), clearing the environment is unnecessary security
theater. The real protection is the mocking layer, not environment isolation.

Benefits of this change:
- Fixes ~55 test failures on Windows (git command not found)
- Allows debugging with environment variables (AIDER_VERBOSE=1, etc.)
- Simpler fixture without platform-specific whitelisting
- Relies on existing mock boundaries for API call prevention

All 104 tests passing on macOS.

Author: johbo

tests/basic/test_main.py

@@ -63,18 +63,18 @@ def test_env(mocker, temp_cwd, temp_home):
 
     All resources are automatically cleaned up by dependency fixtures and mocker.
     """
-    clean_env = {
+    test_env_vars = {
         "OPENAI_API_KEY": "deadbeef",
         "AIDER_CHECK_UPDATE": "false",
         "AIDER_ANALYTICS": "false",
     }
 
     if platform.system() == "Windows":
-        clean_env["USERPROFILE"] = temp_home
+        test_env_vars["USERPROFILE"] = temp_home
     else:
-        clean_env["HOME"] = temp_home
+        test_env_vars["HOME"] = temp_home
 
-    mocker.patch.dict(os.environ, clean_env, clear=True)
+    mocker.patch.dict(os.environ, test_env_vars)
     mocker.patch("builtins.input", return_value=None)
     mocker.patch("aider.io.webbrowser.open")