coach: keep cancelled-turn markers out of summary and fact folds

The rolling-summary and fact-extraction workers read message bodies
directly, so an empty cancelled marker would ride their prompts as a
blank assistant line. Skip empty bodies in both folds. Refs #781

src/lib/ai/coach/conversation-summary.ts

@@ -182,6 +182,9 @@ export async function refreshConversationSummary(
         // fail-closed like every other decrypt in the Coach stack.
         return [];
       }
+      // #781 — skip empty bodies: a cancelled-turn marker (an empty
+      // assistant row closing an aborted turn) carries no content to fold.
+      if (!content.trim()) return [];
       return [{ role: m.role === "assistant" ? "assistant" : "user", content }];
     });
 

src/lib/ai/coach/facts.ts

@@ -255,6 +255,9 @@ async function loadRecentTurns(
   for (const m of [...row.messages].reverse()) {
     const content = decryptOrNull(m.encryptedContent);
     if (content === null) continue;
+    // #781 — skip empty bodies: a cancelled-turn marker (an empty assistant
+    // row closing an aborted turn) carries nothing to extract from.
+    if (!content.trim()) continue;
     turns.push({ role: m.role, content });
   }
   return turns;