Rework context insertion UX (#12360)

- Confirming a completion now runs the command immediately
- Hitting `enter` on a line with a command now runs it
- The output of commands gets folded away and replaced with a custom
placeholder
- Eliminated ambient context

<img width="1588" alt="image"
src="https://github.com/zed-industries/zed/assets/482957/b1927a45-52d6-4634-acc9-2ee539c1d89a">

Release Notes:

- N/A

---------

Co-authored-by: Nathan Sobo <nathan@zed.dev>

crates/assistant_slash_command/Cargo.toml

@@ -15,7 +15,7 @@ path = "src/assistant_slash_command.rs"
 anyhow.workspace = true
 collections.workspace = true
 derive_more.workspace = true
-futures.workspace = true
 gpui.workspace = true
 language.workspace = true
 parking_lot.workspace = true
+workspace.workspace = true