[FEATURE]: agent descriptions in tag autocompletion

[assagman]

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Enhance the agent selection autocomplete by displaying agent descriptions alongside the agent names.

This helps users:

understand what each agent does before picking one,
differentiate agents from local files.

reference PR: #5804

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• agent description used in /agent command are cutoff #4825: agent description used in /agent command are cutoff

Both issues are addressing the need to display agent descriptions more effectively - #4825 specifically mentions the difficulty of reading full descriptions in the /agents command, while this issue focuses on showing descriptions in tag autocompletion. These could be addressed together as part of a broader improvement to agent description visibility.

Feel free to ignore if your specific use case differs from those issues.