Closes AI-261

This PR adds mention disambiguation in the agent panel, which works both
for regular @-mentions as well as for skills. Effectively, when you
mention files with the same name, the mention crease displays the next
path parent name, following a similar approach to common tabs in the
editor. For skills, the skill source is displayed (either global or from
some project).

<img width="500" alt="Screenshot 2026-05-15 at 6  32@2x"
src="https://github.com/user-attachments/assets/72c99480-cf32-42ac-9304-3f65167a1d94"
/>

Release Notes:

- Agent: Improved file and skill mention disambiguation in the agent
panel.

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Co-authored-by: Richard Feldman <richard@zed.dev>