[AGILE-361] Document the selection capability

Sortable-lists opt-in batch selection now has a home in AGENTS.md,
including the item/card attribute split and the fact that batch
movement is not yet implemented, so the next agent building on it does
not have to rediscover either from the controller source.

Author: myabc

frontend/AGENTS.md

@@ -7,6 +7,8 @@
   - `./src/common/` - Framework-agnostic modules (the `core-common` alias), importable from both Angular and Stimulus. Code belongs here when it depends on neither framework and both sides need it; a helper only Stimulus controllers use belongs in `./src/stimulus/helpers/` instead.
   - `./src/stimulus/` - Stimulus controllers
   - `./src/turbo/` - Turbo integration
+- `sortable-lists` batch selection is opt-in: a root enables it with a `selectionEnabled` value, and no other consumer's behavior changes. A root also sets `announcementScope`, so the shared controller's announcements speak the consumer's vocabulary instead of "item", and `selectionDescriptionId`, pointing at one shared element every selected card references via `aria-describedby`. Items declare `movable`, which gates dragging, selection eligibility, and positional moves alike. The pure selection model lives in `./src/common/batch-selection.ts` (framework-agnostic, so Angular consumers can adopt it); the DOM-facing adapter is `sortable-lists/selection.ts`. Batch movement is not implemented: a drag still moves one card and collapses any wider selection onto it — that's a later work package.
+- `data-batch-selected` is written on the sortable item element — the row, in Backlogs — while `aria-current` is written on the card inside it. A stylesheet assuming both live on the same element will silently paint nothing while attribute assertions stay green.
 
 ## Configuration Files