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/src/assistant.rs

@@ -1,17 +1,15 @@
-mod ambient_context;
 pub mod assistant_panel;
 pub mod assistant_settings;
 mod codegen;
 mod completion_provider;
-mod omit_ranges;
 mod prompts;
 mod saved_conversation;
 mod search;
 mod slash_command;
 mod streaming_diff;
 
-use ambient_context::AmbientContextSnapshot;
 pub use assistant_panel::AssistantPanel;
+
 use assistant_settings::{AnthropicModel, AssistantSettings, OpenAiModel, ZedDotDevModel};
 use client::{proto, Client};
 use command_palette_hooks::CommandPaletteFilter;
@@ -38,7 +36,8 @@ actions!(
         InsertActivePrompt,
         ToggleIncludeConversation,
         ToggleHistory,
-        ApplyEdit
+        ApplyEdit,
+        ConfirmCommand
     ]
 );
 
@@ -188,9 +187,6 @@ pub struct LanguageModelChoiceDelta {
 struct MessageMetadata {
     role: Role,
     status: MessageStatus,
-    // TODO: Delete this
-    #[serde(skip)]
-    ambient_context: AmbientContextSnapshot,
 }
 
 #[derive(Clone, Debug, Serialize, Deserialize)]