🤖 fix: add context wrapper to post-compaction follow-up message

After compaction completes, the queued follow-up message was sent as raw
text without any context. This made it unclear to the model that this
message triggered compaction and that relevant context is in the summary.

The fix wraps non-default follow-up messages with a context note:
  "(This was the message that triggered compaction - context is in the
   summary above)"

The default resume sentinel ("Continue") is excluded since it's just a
signal to continue, not meaningful user content.

This is NOT a regression - the original implementation (dacc278, Nov 2025)
always sent follow-ups as raw text. This change improves upon that design.

Author: ammar-agent

src/node/services/agentSession.ts

@@ -595,7 +595,20 @@ export class AgentSession {
       const continueMessage = typedMuxMetadata.parsed.continueMessage;
 
       // Process the continue message content (handles reviews -> text formatting + metadata)
-      const { finalText, metadata } = prepareUserMessageForSend(continueMessage);
+      const prepared = prepareUserMessageForSend(continueMessage);
+      const { metadata } = prepared;
+      let { finalText } = prepared;
+
+      // Wrap non-default follow-up with context so the model knows this was the message
+      // that triggered compaction. The "Continue" sentinel is excluded since it's just
+      // a resume signal, not meaningful user content.
+      const isDefaultResume =
+        continueMessage.text?.trim() === "Continue" &&
+        !continueMessage.imageParts?.length &&
+        !continueMessage.reviews?.length;
+      if (!isDefaultResume && finalText.trim().length > 0) {
+        finalText = `(This was the message that triggered compaction - context is in the summary above)\n\n${finalText}`;
+      }
 
       // Legacy compatibility: older clients stored `continueMessage.mode` (exec/plan) and compaction
       // requests run with agentId="compact". Avoid falling back to the compact agent for the