agent: Less disruptive changed file notification (#31693)

When the user edits one of the tracked files, we used to notify the
agent by inserting a user message at the end of the thread. This was
causing a few problems:
- The agent would stop doing its work and start reading changed files
- The agent would write something like, "Thank you for letting me know
about these changed files."

This fix contains two parts:
1. Changing the prompt to indicate this is a service message
2. Moving the message higher in the conversation thread

This works, but it slightly hurts caching.

We may consider making these notification messages stick in history,
trading context tokens count for the cache.

This might be related to #30906

Release Notes:

- N/A

---------

Co-authored-by: Marshall Bowers <git@maxdeviant.com>

crates/assistant_tool/src/action_log.rs

@@ -456,18 +456,18 @@ impl ActionLog {
         })?
     }
 
-    /// Track a buffer as read, so we can notify the model about user edits.
+    /// Track a buffer as read by agent, so we can notify the model about user edits.
     pub fn buffer_read(&mut self, buffer: Entity<Buffer>, cx: &mut Context<Self>) {
         self.track_buffer_internal(buffer, false, cx);
     }
 
-    /// Mark a buffer as edited, so we can refresh it in the context
+    /// Mark a buffer as created by agent, so we can refresh it in the context
     pub fn buffer_created(&mut self, buffer: Entity<Buffer>, cx: &mut Context<Self>) {
         self.edited_since_project_diagnostics_check = true;
         self.track_buffer_internal(buffer.clone(), true, cx);
     }
 
-    /// Mark a buffer as edited, so we can refresh it in the context
+    /// Mark a buffer as edited by agent, so we can refresh it in the context
     pub fn buffer_edited(&mut self, buffer: Entity<Buffer>, cx: &mut Context<Self>) {
         self.edited_since_project_diagnostics_check = true;