refactor: extract focused clients from GiteaClient (Slices 1-6)

- Create gitea/issues_client.py with IssuesClient class (9 methods)
- Create gitea/prs_client.py with PullRequestsClient class (17 methods)
- Create gitea/files_client.py with FilesClient class (4 methods)
- Create gitea/notifications_client.py with NotificationsClient class (2 methods)
- Create gitea/repos_client.py with ReposClient class (2 methods)
- Create gitea/__init__.py to export all client classes
- Remove delegation methods from GiteaClient (now ~70 lines)
- Update all callers to use sub-clients (client.issues, client.prs, etc.)
- Update test files to mock sub-client attributes

GiteaClient is now a facade that provides access to focused sub-clients:
- repos: Repository operations (ReposClient)
- issues: Issue operations (IssuesClient)
- prs: Pull request operations (PullRequestsClient)
- files: File and git ref operations (FilesClient)
- notifications: Notification operations (NotificationsClient)

Refs: #godclass-refactor

gitea/__init__.py

@@ -0,0 +1,35 @@
+"""Gitea API client package."""
+
+from .client import GiteaClient
+from .files_client import FilesClient
+from .issues_client import IssuesClient
+from .notifications_client import NotificationsClient
+from .prs_client import PullRequestsClient
+from .repos_client import ReposClient
+from .models import (
+    CommentModel,
+    GiteaConfig,
+    IssueModel,
+    LabelModel,
+    PullRequestFileModel,
+    PullRequestModel,
+    RepositoryModel,
+    UserModel,
+)
+
+__all__ = [
+    "FilesClient",
+    "GiteaClient",
+    "IssuesClient",
+    "NotificationsClient",
+    "PullRequestsClient",
+    "ReposClient",
+    "CommentModel",
+    "GiteaConfig",
+    "IssueModel",
+    "LabelModel",
+    "PullRequestFileModel",
+    "PullRequestModel",
+    "RepositoryModel",
+    "UserModel",
+]