remove cursor from MCP tool interface
- cursor is an implementation detail, not useful for LLM workflows
- LLMs don't naturally paginate - they want all results or a filtered subset
- keep cursor parameter in internal functions for future use
- remove from:
- list_repo_branches tool signature
- list_repo_issues tool signature
- ListBranchesResult response type
- ListIssuesResult response type
- update tests to not check cursor field