Add a slash command for automatically retrieving relevant context (#17972)

* [x] put this slash command behind a feature flag until we release
embedding access to the general population
* [x] choose a name for this slash command and name the rust module to
match

Release Notes:

- N/A

---------

Co-authored-by: Jason <jason@zed.dev>
Co-authored-by: Richard <richard@zed.dev>
Co-authored-by: Jason Mancuso <7891333+jvmncs@users.noreply.github.com>
Co-authored-by: Richard Feldman <oss@rtfeldman.com>

crates/assistant/src/prompts.rs

@@ -40,6 +40,11 @@ pub struct TerminalAssistantPromptContext {
     pub user_prompt: String,
 }
 
+#[derive(Serialize)]
+pub struct ProjectSlashCommandPromptContext {
+    pub context_buffer: String,
+}
+
 /// Context required to generate a workflow step resolution prompt.
 #[derive(Debug, Serialize)]
 pub struct StepResolutionContext {
@@ -317,4 +322,14 @@ impl PromptBuilder {
     pub fn generate_workflow_prompt(&self) -> Result<String, RenderError> {
         self.handlebars.lock().render("edit_workflow", &())
     }
+
+    pub fn generate_project_slash_command_prompt(
+        &self,
+        context_buffer: String,
+    ) -> Result<String, RenderError> {
+        self.handlebars.lock().render(
+            "project_slash_command",
+            &ProjectSlashCommandPromptContext { context_buffer },
+        )
+    }
 }