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>

README.md

@@ -74,7 +74,7 @@ GITEA_TOKEN=<bot-personal-access-token>
 
 This does **not** weaken per-user security. OAuth remains authoritative: before every repository call the server verifies that the signed-in user has permission on the target repo through Gitea (`_verify_user_repository_access`) and denies it otherwise. The PAT only performs the API call after that check; OAuth provides identity, per-user authorization, and audit attribution.
 
-Note: with a service PAT, `list_repositories` is intentionally blocked because it has no repository target to authorize per user — use the repository-scoped tools (`get_repository_info`, `get_file_contents`, `list_issues`, …) instead.
+Note: with a service PAT, `list_repositories` is **scoped to the signed-in user** — it returns only the repositories that user owns or contributes to (resolved via Gitea's repo search with the `uid` filter), not everything the bot can see. Visibility of private repos still depends on what the service token itself can access. All other tools require an explicit `owner`/`repo` and run the per-user permission check first.
 
 ### 2a) Required writable volumes (read-only container)