Fix compaction bug: use User role for summary to maintain alternation

The previous implementation added the summary as a System message, which
caused "Conversation must start with a user message" errors because the
first non-system message after compaction was Assistant (the preserved
last assistant message).

Fix: Change summary from System to User message, creating valid alternation:
[System Prompt] -> [Summary as USER] -> [Last Assistant] -> [Latest User]

This also prevents system message bloat across multiple compactions since
the summary is now part of the conversation flow and gets replaced on
each compaction.

Added test_second_compaction_no_bloat to verify no accumulation.

crates/g3-core/src/context_window.rs

@@ -292,9 +292,11 @@ Format this as a detailed but concise summary that can be used to resume the con
             self.add_message(Message::new(MessageRole::System, stub_content));
         }
 
-        // Add the summary
+        // Add the summary as a USER message (not System) to maintain proper alternation.
+        // This allows: [Summary as User] -> [Last Assistant] -> [Latest User]
+        // which is valid User/Assistant alternation.
         self.add_message(Message::new(
-            MessageRole::System,
+            MessageRole::User,
             format!("Previous conversation summary:\n\n{}", summary),
         ));