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_editor.rs

@@ -580,6 +580,7 @@ impl ContextEditor {
                 });
             }
             ContextEvent::SummaryGenerated => {}
+            ContextEvent::PathChanged { .. } => {}
             ContextEvent::StartedThoughtProcess(range) => {
                 let creases = self.insert_thought_process_output_sections(
                     [(