feat: scope list_repositories to the authenticated user in service-PAT mode

Previously list_repositories was blocked in service-PAT mode because it has no
repository target for the per-user permission check, so users could not list
their repositories at all (the connector surfaced a generic error).

list_repositories now returns only the repositories the signed-in user owns or
contributes to, instead of everything the bot token can see:
- gitea_client.py: add list_user_repositories(login) — resolves the user id and
  queries /api/v1/repos/search with the uid filter.
- repository.py: list_repositories_tool uses the user-scoped path when a service
  PAT is configured and a user login is present; pure-OAuth mode still uses the
  user's own /user/repos.
- server.py: allow list_repositories through the service-PAT guard (it is scoped
  to the user in the handler); all other tools still require a repository target.
- README.md: document the new user-scoped behavior and its visibility caveat.

Tests: user-scoped client method (uid resolution + unknown user), PAT-mode tool
scoping, and conftest now clears the request context between tests to prevent
contextvar login leakage across files.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/aegis_gitea_mcp/server.py

@@ -1165,26 +1165,32 @@ async def _execute_tool_call(
 
         if settings.gitea_token.strip():
             if not repository:
-                audit.log_access_denied(
-                    tool_name=tool_name,
-                    reason="service_pat_requires_repository_target",
+                # list_repositories is not repo-scoped; the handler scopes it to
+                # the authenticated user's own repositories instead. Every other
+                # tool requires a repository target so per-user permission can be
+                # verified before the privileged service PAT is used.
+                if tool_name != "list_repositories":
+                    audit.log_access_denied(
+                        tool_name=tool_name,
+                        reason="service_pat_requires_repository_target",
+                        correlation_id=correlation_id,
+                    )
+                    raise HTTPException(
+                        status_code=403,
+                        detail=(
+                            "Service PAT mode requires a repository target so per-user "
+                            "permission can be verified."
+                        ),
+                    )
+            else:
+                user_login = get_gitea_user_login()
+                await _verify_user_repository_access(
+                    repository=repository,
+                    required_scope=required_scope,
+                    user_login=user_login or "",
                     correlation_id=correlation_id,
+                    tool_name=tool_name,
                 )
-                raise HTTPException(
-                    status_code=403,
-                    detail=(
-                        "Service PAT mode requires a repository target so per-user "
-                        "permission can be verified."
-                    ),
-                )
-            user_login = get_gitea_user_login()
-            await _verify_user_repository_access(
-                repository=repository,
-                required_scope=required_scope,
-                user_login=user_login or "",
-                correlation_id=correlation_id,
-                tool_name=tool_name,
-            )
 
         # In OAuth mode, Gitea OIDC access_tokens can't call the Gitea REST API
         # (they only carry OIDC scopes).  If a service PAT is configured via