ci: Strip duplicate PR numbers and simplify mentions from changelog

[DylanDevelops]

Pull Request Checklist

• I have read and followed the contribution guidelines.
• My pull request targets the main branch of tmpo.
• I have tested these changes locally on my machine.

Description

This pull request updates the cliff.toml changelog template to improve the formatting and clarity of contributor and commit information. The main changes focus on simplifying the display of usernames and pull request references.

Changelog formatting improvements:

• Commit titles now have pull request numbers removed from the end if present, preventing duplication when the PR number is shown separately.
• Usernames are now displayed as plain text (e.g., @username) instead of markdown links, both in the commit attributions and the new contributors section. [1] [2]
• The "New Contributors" section now lists contributors as @username made their first contribution! instead of linking their usernames.