Improve Zed agent thread history to update across all open windows (#47803)

Previously, each Agent panel created its own `ThreadStore` instance, so
thread history updates wouldn't sync between open windows. Users had to
close and reopen windows to see threads created in other windows.

Now `ThreadStore` is initialized once as a global and shared across all
windows. When any window saves a thread, ThreadStore::reload() calls
cx.notify(), which notifies all NativeAgentSessionList observers,
causing all windows' history views to refresh.

Note: This only works for the native Zed agent. It also is only
improving the history view (thread titles)—this PR does not sync edits
between multiple windows when the same thread is open in each.

Release Notes:

- Improved Zed agent thread history to update across all open windows

crates/agent/src/thread_store.rs

@@ -1,10 +1,14 @@
 use crate::{DbThread, DbThreadMetadata, ThreadsDatabase};
 use agent_client_protocol as acp;
 use anyhow::{Result, anyhow};
-use gpui::{App, Context, Entity, Task, prelude::*};
+use gpui::{App, Context, Entity, Global, Task, prelude::*};
 use project::Project;
 use std::rc::Rc;
 
+struct GlobalThreadStore(Entity<ThreadStore>);
+
+impl Global for GlobalThreadStore {}
+
 // TODO: Remove once ACP thread loading is fully handled elsewhere.
 pub fn load_agent_thread(
     session_id: acp::SessionId,
@@ -37,6 +41,15 @@ pub struct ThreadStore {
 }
 
 impl ThreadStore {
+    pub fn init_global(cx: &mut App) {
+        let thread_store = cx.new(|cx| Self::new(cx));
+        cx.set_global(GlobalThreadStore(thread_store));
+    }
+
+    pub fn global(cx: &App) -> Entity<Self> {
+        cx.global::<GlobalThreadStore>().0.clone()
+    }
+
     pub fn new(cx: &mut Context<Self>) -> Self {
         let this = Self {
             threads: Vec::new(),

crates/agent_ui/src/agent_panel.rs

@@ -507,7 +507,7 @@ impl AgentPanel {
         let context_server_registry =
             cx.new(|cx| ContextServerRegistry::new(project.read(cx).context_server_store(), cx));
 
-        let thread_store = cx.new(|cx| ThreadStore::new(cx));
+        let thread_store = ThreadStore::global(cx);
         let acp_history = cx.new(|cx| AcpThreadHistory::new(None, window, cx));
         let text_thread_history =
             cx.new(|cx| TextThreadHistory::new(text_thread_store.clone(), window, cx));

crates/agent_ui/src/agent_ui.rs

@@ -256,6 +256,7 @@ pub fn init(
     is_eval: bool,
     cx: &mut App,
 ) {
+    agent::ThreadStore::init_global(cx);
     assistant_text_thread::init(client, cx);
     rules_library::init(cx);
     if !is_eval {

crates/agent_ui_v2/src/agents_panel.rs

@@ -124,7 +124,7 @@ impl AgentsPanel {
     ) -> Self {
         let focus_handle = cx.focus_handle();
 
-        let thread_store = cx.new(|cx| ThreadStore::new(cx));
+        let thread_store = ThreadStore::global(cx);
         let history = cx.new(|cx| AcpThreadHistory::new(None, window, cx));
 
         let history_handle = history.clone();