Allow prompt templates to be overridden in the zed configuration directory (#15887)

I need this to refine our prompts on the fly as I work.

Release Notes:

- Templates for prompts driving inline transformation in editors and the
terminal can now be overridden in the `~/.config/zed/prompts/templates`
directory. This is an advanced feature, and prevents you from getting
upstream changes. It's intended for use by Zed developers.

crates/assistant/src/assistant.rs

@@ -35,6 +35,7 @@ use slash_command::{
 };
 use std::sync::Arc;
 pub(crate) use streaming_diff::*;
+use util::ResultExt;
 
 actions!(
     assistant,
@@ -197,8 +198,17 @@ pub fn init(fs: Arc<dyn Fs>, client: Arc<Client>, cx: &mut AppContext) {
     assistant_slash_command::init(cx);
     register_slash_commands(cx);
     assistant_panel::init(cx);
-    inline_assistant::init(fs.clone(), client.telemetry().clone(), cx);
-    terminal_inline_assistant::init(fs.clone(), client.telemetry().clone(), cx);
+
+    if let Some(prompt_builder) = prompts::PromptBuilder::new(Some((fs.clone(), cx))).log_err() {
+        let prompt_builder = Arc::new(prompt_builder);
+        inline_assistant::init(
+            fs.clone(),
+            prompt_builder.clone(),
+            client.telemetry().clone(),
+            cx,
+        );
+        terminal_inline_assistant::init(fs.clone(), prompt_builder, client.telemetry().clone(), cx);
+    }
     IndexedDocsRegistry::init_global(cx);
 
     CommandPaletteFilter::update_global(cx, |filter, _cx| {