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/tools/git_tools.py

@@ -10,7 +10,7 @@ class GitTools:
 
     def create_branch(self, owner: str, repo: str, ref: str, sha: str) -> str:
         try:
-            self._client.create_ref(owner, repo, ref, sha)
+            self._client.files.create_ref(owner, repo, ref, sha)
             return f"Branch '{ref}' created successfully in {owner}/{repo}."
         except Exception as e:
             return f"Error creating branch: {str(e)}"