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/agent/src/context_picker/thread_context_picker.rs

@@ -282,15 +282,18 @@ pub fn unordered_thread_entries(
     text_thread_store: Entity<TextThreadStore>,
     cx: &App,
 ) -> impl Iterator<Item = (DateTime<Utc>, ThreadContextEntry)> {
-    let threads = thread_store.read(cx).unordered_threads().map(|thread| {
-        (
-            thread.updated_at,
-            ThreadContextEntry::Thread {
-                id: thread.id.clone(),
-                title: thread.summary.clone(),
-            },
-        )
-    });
+    let threads = thread_store
+        .read(cx)
+        .reverse_chronological_threads()
+        .map(|thread| {
+            (
+                thread.updated_at,
+                ThreadContextEntry::Thread {
+                    id: thread.id.clone(),
+                    title: thread.summary.clone(),
+                },
+            )
+        });
 
     let text_threads = text_thread_store
         .read(cx)
@@ -300,7 +303,7 @@ pub fn unordered_thread_entries(
                 context.mtime.to_utc(),
                 ThreadContextEntry::Context {
                     path: context.path.clone(),
-                    title: context.title.clone().into(),
+                    title: context.title.clone(),
                 },
             )
         });