fix(herdr): support Herdr 0.8.0 pane allocation, agent prompt, and length limits

tests/test_b8_send_keys_verification.py

@@ -15,7 +15,12 @@ _pane_quiescent() {{ return 0; }}
 _pane_dialog_open() {{ return 1; }}
 
 # Mock _sks_herdr commands
-_sks_herdr() {{ return 0; }}
+_sks_herdr() {{
+  if [ "${{1:-}}" = "agent" ] && [ "${{2:-}}" = "prompt" ]; then
+    return 1
+  fi
+  return 0
+}}
 
 # Mock _pane_capture to simulate fixed pane content (no execution token, no pane change)
 _pane_capture() {{ echo "static content"; }}