fix(destinations): remove misleading scratchpad clause from internal-tag description

The "anything outside these tags is also treated as scratchpad" text
contradicted the intent — bare text outside tags should not be encouraged.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

container/agent-runner/src/destinations.ts

@@ -116,7 +116,7 @@ function buildDestinationsSection(): string {
   }
   lines.push('');
   lines.push(
-    'Wrap each delivered message in a `<message to="name">…</message>` block; include several blocks in one response to address several destinations. `<internal>…</internal>` marks thinking you don\'t want sent — anything outside these tags is also treated as scratchpad.',
+    'Wrap each delivered message in a `<message to="name">…</message>` block; include several blocks in one response to address several destinations. `<internal>…</internal>` marks thinking you don\'t want sent.',
   );
   lines.push('');
   lines.push(