ai onboarding: Add overall fixes to the whole flow (#34996)

Closes https://github.com/zed-industries/zed/issues/34979

Release Notes:

- N/A

---------

Co-authored-by: Agus Zubiaga <hi@aguz.me>
Co-authored-by: Ben Kunkle <Ben.kunkle@gmail.com>

crates/agent/src/thread_store.rs

@@ -41,6 +41,9 @@ use std::{
 };
 use util::ResultExt as _;
 
+pub static ZED_STATELESS: std::sync::LazyLock<bool> =
+    std::sync::LazyLock::new(|| std::env::var("ZED_STATELESS").map_or(false, |v| !v.is_empty()));
+
 #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
 pub enum DataType {
     #[serde(rename = "json")]
@@ -874,7 +877,11 @@ impl ThreadsDatabase {
 
         let needs_migration_from_heed = mdb_path.exists();
 
-        let connection = Connection::open_file(&sqlite_path.to_string_lossy());
+        let connection = if *ZED_STATELESS {
+            Connection::open_memory(Some("THREAD_FALLBACK_DB"))
+        } else {
+            Connection::open_file(&sqlite_path.to_string_lossy())
+        };
 
         connection.exec(indoc! {"
                 CREATE TABLE IF NOT EXISTS threads (