@mention autocompletion

@mentions are a feature designed to improve collaboration on GitHub by allowing users to easily include others on issues or pull requests through an autocompleter that activates by typing the '@' symbol. This tool helps identify and select collaborators or participants within a specific repository, enhancing communication without needing a global search. Users can filter and choose the correct person from a list by typing or using arrow keys, making the process efficient and focused on relevant people. The @mention feature, which also works for usernames and full names, is available in various GitHub contexts, including blog comments, and aims to streamline the process of involving others in discussions or tasks.