feat(gitea): add CRUD operations for Issues and Pull Requests
Implements missing functions from issue #3:
- Issues: get_issue, update_issue, close_issue, reopen_issue, delete_issue
- Issue Comments: list_issue_comments, create_issue_comment, update_issue_comment, delete_issue_comment
- Pull Requests: get_pull_request, update_pull_request, merge_pull_request
- PR Comments: list_pull_request_comments, create_pull_request_comment, update_pull_request_comment, delete_pull_request_comment
Refs: #3
xcaliber
f19a229025