Use the same prompt as agent thread summary for text threads (#35669)

This was causing text thread summarization to be counted as a usage of 1
prompt

Release Notes:

- Fixed bug with agent text threads (not chat threads) counting
summarization as a usage of 1 prompt.

Co-authored-by: Oleksiy <oleksiy@zed.dev>

crates/agent_settings/src/agent_settings.rs

@@ -13,6 +13,9 @@ use std::borrow::Cow;
 
 pub use crate::agent_profile::*;
 
+pub const SUMMARIZE_THREAD_PROMPT: &str =
+    include_str!("../../agent/src/prompts/summarize_thread_prompt.txt");
+
 pub fn init(cx: &mut App) {
     AgentSettings::register(cx);
 }