Closes #ISSUE
Adds a new `documentation` method to actions, that is extracted from doc
comments when using the `actions!` or derive macros.
Additionally, this PR adds doc comments to as many action definitions in
Zed as possible.
Release Notes:
- N/A *or* Added/Fixed/Improved ...
This breaks a transitive dependency of `agent` on UI crates. I've also
found and eliminated some dead code in assistant_context_editor.
Release Notes:
- N/A