feat: allow relative target paths for git mounts

- Remove absolute path requirement from target_path validation
- Resolve relative paths against working_directory at instance startup
- Fall back to /home/user if no working_directory is configured
- Update frontend to allow relative target paths
- Update spec to document relative path support
- Update tests to allow relative paths and test path traversal rejection

apps/api/tests/integration/test_config_profiles_api.py

@@ -402,8 +402,8 @@ class TestConfigProfilesAPI:
         )
         assert response.status_code == 422
 
-    def test_create_config_profile_invalid_git_mount_target_path(self, authenticated_client: TestClient, test_project_and_repo) -> None:
-        """Test that invalid git mount target paths are rejected."""
+    def test_create_config_profile_invalid_git_mount_target_path_traversal(self, authenticated_client: TestClient, test_project_and_repo) -> None:
+        """Test that git mount target paths with traversal are rejected."""
         _project_id, repo_id = test_project_and_repo
 
         response = authenticated_client.post(
@@ -416,7 +416,7 @@ class TestConfigProfilesAPI:
                     {
                         "remote_url": "https://github.com/user/repo.git",
                         "source_path": ".",
-                        "target_path": "relative/path",
+                        "target_path": "../../../etc/passwd",
                     }
                 ],
             },