acp: Support session modes (e.g. CC plan mode) (#37632)

Adds support for [ACP session
modes](https://github.com/zed-industries/agent-client-protocol/pull/67)
enabling plan and other permission modes in CC:


https://github.com/user-attachments/assets/dea18d82-4da6-465e-983b-02b77c6dcf15


Release Notes:

- Claude Code: Add support for plan mode, and all other permission modes

---------

Co-authored-by: Bennet Bo Fenner <bennetbo@gmx.de>
Co-authored-by: Richard Feldman <oss@rtfeldman.com>
Co-authored-by: Danilo Leal <daniloleal09@gmail.com>

crates/agent_servers/src/custom.rs

@@ -1,9 +1,12 @@
 use crate::AgentServerDelegate;
 use acp_thread::AgentConnection;
+use agent_client_protocol as acp;
 use anyhow::{Context as _, Result};
-use gpui::{App, SharedString, Task};
-use project::agent_server_store::ExternalAgentServerName;
-use std::{path::Path, rc::Rc};
+use fs::Fs;
+use gpui::{App, AppContext as _, SharedString, Task};
+use project::agent_server_store::{AllAgentServersSettings, ExternalAgentServerName};
+use settings::{SettingsStore, update_settings_file};
+use std::{path::Path, rc::Rc, sync::Arc};
 use ui::IconName;
 
 /// A generic agent server implementation for custom user-defined agents
@@ -30,6 +33,27 @@ impl crate::AgentServer for CustomAgentServer {
         IconName::Terminal
     }
 
+    fn default_mode(&self, cx: &mut App) -> Option<acp::SessionModeId> {
+        let settings = cx.read_global(|settings: &SettingsStore, _| {
+            settings
+                .get::<AllAgentServersSettings>(None)
+                .custom
+                .get(&self.name())
+                .cloned()
+        });
+
+        settings
+            .as_ref()
+            .and_then(|s| s.default_mode.clone().map(|m| acp::SessionModeId(m.into())))
+    }
+
+    fn set_default_mode(&self, mode_id: Option<acp::SessionModeId>, fs: Arc<dyn Fs>, cx: &mut App) {
+        let name = self.name();
+        update_settings_file::<AllAgentServersSettings>(fs, cx, move |settings, _| {
+            settings.custom.get_mut(&name).unwrap().default_mode = mode_id.map(|m| m.to_string())
+        });
+    }
+
     fn connect(
         &self,
         root_dir: Option<&Path>,
@@ -39,6 +63,7 @@ impl crate::AgentServer for CustomAgentServer {
         let name = self.name();
         let root_dir = root_dir.map(|root_dir| root_dir.to_string_lossy().to_string());
         let is_remote = delegate.project.read(cx).is_via_remote_server();
+        let default_mode = self.default_mode(cx);
         let store = delegate.store.downgrade();
 
         cx.spawn(async move |cx| {
@@ -58,8 +83,15 @@ impl crate::AgentServer for CustomAgentServer {
                     ))
                 })??
                 .await?;
-            let connection =
-                crate::acp::connect(name, command, root_dir.as_ref(), is_remote, cx).await?;
+            let connection = crate::acp::connect(
+                name,
+                command,
+                root_dir.as_ref(),
+                default_mode,
+                is_remote,
+                cx,
+            )
+            .await?;
             Ok((connection, login))
         })
     }