refactor(gists): migrate gists.go to NewTool pattern

Convert all gist tools (ListGists, GetGist, CreateGist, UpdateGist)
to use the new NewTool helper with ToolDependencies injection.

- Remove getClient parameter from function signatures
- Use deps.GetClient(ctx) inside handlers
- Standardize error handling with utils.NewToolResultErrorFromErr()
- Update all tests to use serverTool.Handler(deps) pattern

Co-authored-by: Adam Holt <oholt@github.com>

Author: SamMorrowDrums