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/src/thread.rs

@@ -8,7 +8,7 @@ use crate::{
     },
     tool_use::{PendingToolUse, ToolUse, ToolUseMetadata, ToolUseState},
 };
-use agent_settings::{AgentProfileId, AgentSettings, CompletionMode};
+use agent_settings::{AgentProfileId, AgentSettings, CompletionMode, SUMMARIZE_THREAD_PROMPT};
 use anyhow::{Result, anyhow};
 use assistant_tool::{ActionLog, AnyToolCard, Tool, ToolWorkingSet};
 use chrono::{DateTime, Utc};
@@ -2112,12 +2112,10 @@ impl Thread {
             return;
         }
 
-        let added_user_message = include_str!("./prompts/summarize_thread_prompt.txt");
-
         let request = self.to_summarize_request(
             &model.model,
             CompletionIntent::ThreadSummarization,
-            added_user_message.into(),
+            SUMMARIZE_THREAD_PROMPT.into(),
             cx,
         );