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/assistant.rs

@@ -34,6 +34,7 @@ use language_model::{
 };
 pub(crate) use model_selector::*;
 pub use prompts::PromptBuilder;
+use prompts::PromptLoadingParams;
 use semantic_index::{CloudEmbeddingProvider, SemanticIndex};
 use serde::{Deserialize, Serialize};
 use settings::{update_settings_file, Settings, SettingsStore};
@@ -180,7 +181,12 @@ impl Assistant {
     }
 }
 
-pub fn init(fs: Arc<dyn Fs>, client: Arc<Client>, cx: &mut AppContext) -> Arc<PromptBuilder> {
+pub fn init(
+    fs: Arc<dyn Fs>,
+    client: Arc<Client>,
+    stdout_is_a_pty: bool,
+    cx: &mut AppContext,
+) -> Arc<PromptBuilder> {
     cx.set_global(Assistant::default());
     AssistantSettings::register(cx);
     SlashCommandSettings::register(cx);
@@ -217,10 +223,16 @@ pub fn init(fs: Arc<dyn Fs>, client: Arc<Client>, cx: &mut AppContext) -> Arc<Pr
     assistant_panel::init(cx);
     context_servers::init(cx);
 
-    let prompt_builder = prompts::PromptBuilder::new(Some((fs.clone(), cx)))
-        .log_err()
-        .map(Arc::new)
-        .unwrap_or_else(|| Arc::new(prompts::PromptBuilder::new(None).unwrap()));
+    let prompt_builder = prompts::PromptBuilder::new(Some(PromptLoadingParams {
+        fs: fs.clone(),
+        repo_path: stdout_is_a_pty
+            .then(|| std::env::current_dir().log_err())
+            .flatten(),
+        cx,
+    }))
+    .log_err()
+    .map(Arc::new)
+    .unwrap_or_else(|| Arc::new(prompts::PromptBuilder::new(None).unwrap()));
     register_slash_commands(Some(prompt_builder.clone()), cx);
     inline_assistant::init(
         fs.clone(),