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

README.md

@@ -93,9 +93,9 @@ batch mode, DO upload/staged/settings, CSV/Excel export). Data source:
 
 | Mode | Who opens/closes batch | DO gate |
 |---|---|---|
-| `auto` (default) | AI truck FSM | — |
+| `auto` | AI truck FSM — merges batches when a sack sits in the counting ROI (known) | — |
 | `do_manual` | Operator + DO photo scan | yes |
-| `manual` | Operator buttons (legacy) | no |
+| `manual` (default) | Operator buttons + plate number per truck | no |
 
 Office (`:5721`) switches mode / model mode / require_plate / ocr_engine (403
 on operator POST). **DO manual flow:** open `http://<jetson>:5000/operator` on a phone →