Closes #ISSUE

Co-authored-by: Bennet <bennet@zed.dev>

Release Notes:

- Added support for context `@mentions` in the inline prompt editor and
when editing past messages in the agent panel.

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Co-authored-by: Bennet Bo Fenner <bennet@zed.dev>
Co-authored-by: Ben Brandt <benjamin.j.brandt@gmail.com>