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/thread_store.rs

@@ -393,16 +393,11 @@ impl ThreadStore {
         self.threads.len()
     }
 
-    pub fn unordered_threads(&self) -> impl Iterator<Item = &SerializedThreadMetadata> {
+    pub fn reverse_chronological_threads(&self) -> impl Iterator<Item = &SerializedThreadMetadata> {
+        // ordering is from "ORDER BY" in `list_threads`
         self.threads.iter()
     }
 
-    pub fn reverse_chronological_threads(&self) -> Vec<SerializedThreadMetadata> {
-        let mut threads = self.threads.iter().cloned().collect::<Vec<_>>();
-        threads.sort_unstable_by_key(|thread| std::cmp::Reverse(thread.updated_at));
-        threads
-    }
-
     pub fn create_thread(&mut self, cx: &mut Context<Self>) -> Entity<Thread> {
         cx.new(|cx| {
             Thread::new(