refactor: Convert all tool names from kebab-case to snake_case

- Changed all 16 MCP tool names to use snake_case per JSON-RPC 2.0 spec requirements
- Tool names are used as JSON-RPC method names and must follow naming conventions
- Updated all references in server.py and test files
- Fixed linting errors: removed unused variables, added strict=False to zip()
- Fixed type annotations for better type safety
- All tests pass with 82% coverage (380 tests)

Tool name changes:
- hm-search → hm_search
- hm-show → hm_show
- hm-stats → hm_stats
- hm-options → hm_options
- hm-browse → hm_browse
- darwin-search → darwin_search
- darwin-show → darwin_show
- darwin-stats → darwin_stats
- darwin-options → darwin_options
- darwin-browse → darwin_browse
- flake-search → flake_search
- find-version → find_version
- discourse-search → discourse_search
- github-search → github_search
- quick-start → quick_start
- try-package → try_package

Author: jamesbrink