Move UI code from assistant_context_editor -> agent_ui (#33289)

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

crates/collab/src/tests/integration_tests.rs

@@ -6,7 +6,7 @@ use crate::{
     },
 };
 use anyhow::{Result, anyhow};
-use assistant_context_editor::ContextStore;
+use assistant_context::ContextStore;
 use assistant_slash_command::SlashCommandWorkingSet;
 use buffer_diff::{DiffHunkSecondaryStatus, DiffHunkStatus, assert_hunks};
 use call::{ActiveCall, ParticipantLocation, Room, room};