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

docs/configuration.md

@@ -36,9 +36,9 @@ deprecated (warned).
 
 | Mode | Who opens/closes | DO gate |
 |---|---|---|
-| `auto` (default) | AI truck FSM | n/a |
+| `auto` | AI truck FSM — merges batches when a sack sits in the counting ROI (known) | n/a |
 | `do_manual` | Operator start/stop + staged DO photos | yes |
-| `manual` | Operator start/stop (legacy) — plate required at start | no |
+| `manual` (**default**) | Operator start/stop, plate required at start | no |
 
 `POST /api/batch/mode` with `mode` and/or `model_mode` is **office port only**
 (403 on operator `:5000`); mode switch while a batch is active → **409**.