Improve dev experience for built-in prompts (#16413)

When launching Zed from the CLI via `cargo run`, we'll always prompt
load templates from the repo.

This restores behavior that I reverted last night in #16403.

Also, I've improved the `script/prompts link/unlink` workflow for
overriding prompts of your production copy of Zed. Zed now detects when
the overrides directory is created or removed, and does the right thing.
You can link and unlink repeatedly without restarting Zed.

Release Notes:

- N/A

crates/assistant/src/context/context_tests.rs

@@ -473,7 +473,7 @@ async fn test_slash_commands(cx: &mut TestAppContext) {
 }
 
 #[gpui::test]
-async fn test_edit_step_parsing(cx: &mut TestAppContext) {
+async fn test_workflow_step_parsing(cx: &mut TestAppContext) {
     cx.update(prompt_library::init);
     let settings_store = cx.update(SettingsStore::test);
     cx.set_global(settings_store);