feat: notify user when context compaction starts and completes (#38805)

Merged via squash.

Prepared head SHA: 0f48c1bbf6
Co-authored-by: zidongdesign <81469543+zidongdesign@users.noreply.github.com>
Co-authored-by: jalehman <550978+jalehman@users.noreply.github.com>
Reviewed-by: @jalehman

src/auto-reply/types.ts

@@ -91,6 +91,10 @@ export type ReplyPayload = {
   /** Marks this payload as a reasoning/thinking block. Channels that do not
    *  have a dedicated reasoning lane (e.g. WhatsApp, web) should suppress it. */
   isReasoning?: boolean;
+  /** Marks this payload as a compaction status notice (start/end).
+   *  Should be excluded from TTS transcript accumulation so compaction
+   *  status lines are not synthesised into the spoken assistant reply. */
+  isCompactionNotice?: boolean;
   /** Channel-specific payload data (per-channel envelope). */
   channelData?: Record<string, unknown>;
 };