The last welded composer engine. The `@`/`/` trigger state, detection
(refreshTrigger), the adapter-driven item list + its effects, popover selection,
closeTrigger, commitTypedSlashDirective, and the contentEditable chip insertion
(replaceTriggerWithChip) move verbatim into hooks/use-composer-trigger.ts behind a
hook that takes the editor refs + the two completion sources (at/slash). ChatBar's
input/keydown/keyup paths + the popover render consume the returned API; the
keydown navigation block stays in place (no key-handling restructure), and
triggerKeyConsumedRef is exposed so keyup still skips its post-consume refresh.
ChatBar 1,248 → 1,047. Behaviour-preserving: typecheck 0 errors, eslint clean,
and the composer DOM repro suite (slash-nav, enter-submit, IME composition,
trigger-popover) is green — the documented IME/caret/focus edge fixes ride along
verbatim. (The 1 attachments.test.tsx failure is pre-existing on main.)
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