fix: prevent assistant message duplication during tool calls

- Fix sync logic to only break at matching assistant messages
- When assistant message changes (streaming → final), properly pop and re-display
- Add debug logging for sync operations (can be removed later)

The bug: when tool calls split an assistant response into multiple entries,
the sync logic was breaking at the assistant even when it didn't match,
causing the old display to remain while new entries were added on top.

The fix: only break at assistant if matches=true, ensuring changed entries
are properly popped before re-adding.

Co-Authored-By: ProofOfConcept <poc@bcachefs.org>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

src/agent/api/types.rs

@@ -103,7 +103,7 @@ pub struct Message {
     pub timestamp: Option<String>,
 }
 
-#[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
+#[derive(Debug, Copy, Clone, Serialize, Deserialize, PartialEq)]
 #[serde(rename_all = "lowercase")]
 pub enum Role {
     System,