feat(manual-batch): canonical plate + manual mode as default

- canonical_plate (uppercase alnum, strips space/dot/hyphen) at all plate
  write sites; pretty_plate for render (history, XLSX, operator tile) so
  B 1234 XYZ and B1234XYZ are one plate everywhere
- group_dos_by_plate compares canonical plates -> no false mixed_plates
- warn-only (never blocking) plate format hint in operator + monitoring modals
- batch.default_mode: manual (auto merged truck loads when a sack sat in the
  counting ROI); operator banner explains plate -> start -> stop
- docs + AGENTS known-limitation note; 85 tests pass

tests/test_config_loader.py

@@ -41,7 +41,7 @@ def test_repo_config_loads_and_validates(repo_config_path):
 
 def test_do_block_and_batch_default_mode(repo_config_path):
     cfg = load_config(repo_config_path)
-    assert cfg.batch.default_mode == "auto"
+    assert cfg.batch.default_mode == "manual"
     # timeout_seconds = sack-idle (pause); truck_gone_tolerance_seconds = finalize
     assert cfg.batch.timeout_seconds == 20.0
     assert cfg.batch.truck_gone_tolerance_seconds == 30.0