Document return value semantics on cancellation (issue #191)

Add comments explaining that Harness::chat() returns an empty string
on Ctrl+C cancellation rather than std::unexpected. This preserves
the conversation state in the chat file, allowing users to resume
the conversation from the point of interruption.

src/harness.cpp

@@ -84,6 +84,15 @@ std::expected<std::string, std::string> Harness::chat(const Message& user_messag
 
         if (ctrlc_request)
         {
+            // Cancellation (Ctrl+C): return empty string to indicate the chat was
+            // interrupted. The user message has already been pushed to history, so
+            // the conversation state will be saved to the chat file. This allows
+            // the user to resume the conversation from this point.
+            //
+            // Note: We return an empty string (success) rather than std::unexpected
+            // because cancellation is not an error condition - it's a deliberate
+            // user action to stop processing. Using std::unexpected would prevent
+            // the harness state from being saved, losing the user's input.
             return "";
         }
 
@@ -93,6 +102,8 @@ std::expected<std::string, std::string> Harness::chat(const Message& user_messag
         while (!done)
         {
             if (ctrlc_request)
+                // Cancellation during API call: same semantics as above - return
+                // empty string to preserve conversation state in the chat file.
                 return "";
 
             retries--;