No longer instantiate recently opened agent threads on startup (#32285)

This was causing a lot of work on startup, particularly due to
instantiating edit tool cards. The minor downside is that now these
threads don't open quite as fast.

Includes a few other improvements:

* On text thread rename, now immediately updates the metadata for
display in the UI instead of waiting for reload.

* On text thread rename, first renames the file before writing. Before
if the file removal failed you'd end up with a duplicate.

* Now only stores text thread file names instead of full paths. This is
more concise and allows for the app data dir changing location.

* Renames `ThreadStore::unordered_threads` to
`ThreadStore::reverse_chronological_threads` (and removes the old one
that sorted), since the recent change to use a SQL database queries them
in that order.

* Removes `ContextStore::reverse_chronological_contexts` since it was
only used in one location where it does sorting anyway - no need to sort
twice.

* `SavedContextMetadata::title` is now `SharedString` instead of
`String`.

Release Notes:

- Fixed regression in startup performance by not deserializing and
instantiating recently opened agent threads.

crates/assistant_context_editor/src/context.rs

@@ -11,7 +11,7 @@ use assistant_slash_commands::FileCommandMetadata;
 use client::{self, proto, telemetry::Telemetry};
 use clock::ReplicaId;
 use collections::{HashMap, HashSet};
-use fs::{Fs, RemoveOptions};
+use fs::{Fs, RenameOptions};
 use futures::{FutureExt, StreamExt, future::Shared};
 use gpui::{
     App, AppContext as _, Context, Entity, EventEmitter, RenderImage, SharedString, Subscription,
@@ -452,6 +452,10 @@ pub enum ContextEvent {
     MessagesEdited,
     SummaryChanged,
     SummaryGenerated,
+    PathChanged {
+        old_path: Option<Arc<Path>>,
+        new_path: Arc<Path>,
+    },
     StreamedCompletion,
     StartedThoughtProcess(Range<language::Anchor>),
     EndedThoughtProcess(language::Anchor),
@@ -2894,22 +2898,34 @@ impl AssistantContext {
                 }
 
                 fs.create_dir(contexts_dir().as_ref()).await?;
-                fs.atomic_write(new_path.clone(), serde_json::to_string(&context).unwrap())
-                    .await?;
-                if let Some(old_path) = old_path {
+
+                // rename before write ensures that only one file exists
+                if let Some(old_path) = old_path.as_ref() {
                     if new_path.as_path() != old_path.as_ref() {
-                        fs.remove_file(
+                        fs.rename(
                             &old_path,
-                            RemoveOptions {
-                                recursive: false,
-                                ignore_if_not_exists: true,
+                            &new_path,
+                            RenameOptions {
+                                overwrite: true,
+                                ignore_if_exists: true,
                             },
                         )
                         .await?;
                     }
                 }
 
-                this.update(cx, |this, _| this.path = Some(new_path.into()))?;
+                // update path before write in case it fails
+                this.update(cx, {
+                    let new_path: Arc<Path> = new_path.clone().into();
+                    move |this, cx| {
+                        this.path = Some(new_path.clone());
+                        cx.emit(ContextEvent::PathChanged { old_path, new_path });
+                    }
+                })
+                .ok();
+
+                fs.atomic_write(new_path, serde_json::to_string(&context).unwrap())
+                    .await?;
             }
 
             Ok(())
@@ -3277,7 +3293,7 @@ impl SavedContextV0_1_0 {
 
 #[derive(Debug, Clone)]
 pub struct SavedContextMetadata {
-    pub title: String,
+    pub title: SharedString,
     pub path: Arc<Path>,
     pub mtime: chrono::DateTime<chrono::Local>,
 }