fix(ai-chat): address PR #211 review (i18n keys, dead export, flag leak)

- Register the new AI-chat keys "Send now" and "Interrupt and send now" in
  both en-US and ru-RU catalogs so the UI never renders mixed-language
  tooltip/aria-label (i18n policy).
- Make INTERRUPT_NOTE module-private (drop the unused re-export), matching the
  module's private DEFAULT_PROMPT/SAFETY_FRAMEWORK siblings.
- Reset interruptNextSendRef in the flush-on-abort branch when nothing is
  actually sent, so a stuck one-shot interrupt flag cannot tag the next
  unrelated send; flushNext now reports whether it sent.
- Add a CHANGELOG [Unreleased]/Added entry for #198.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/server/src/core/ai-chat/ai-chat.prompt.ts

@@ -72,8 +72,6 @@ const INTERRUPT_NOTE =
   'assume your previous response was complete, and do not silently restart the ' +
   'partial work — build on it or follow the new instruction.';
 
-export { INTERRUPT_NOTE };
-
 export interface BuildSystemPromptInput {
   workspace: Workspace;
   /**