feat(background-process): notify sessions when tasks end

Send synthetic session notifications when background processes finish, fail, stop, or terminate so the originating agent can react without polling. Hide synthetic text-only prompts from the UI stream so operational notifications stay out of the visible transcript.

packages/ui/src/components/message-timeline.tsx

@@ -2,6 +2,7 @@ import { For, Show, createEffect, createMemo, createSignal, onCleanup, on, untra
 import MessagePreview from "./message-preview"
 import { messageStoreBus } from "../stores/message-v2/bus"
 import type { ClientPart } from "../types/message"
+import { isHiddenSyntheticTextPart } from "../types/message"
 import type { MessageRecord } from "../stores/message-v2/types"
 import { buildRecordDisplayData } from "../stores/message-v2/record-display-cache"
 import { getPartCharCount } from "../lib/token-utils"
@@ -105,6 +106,7 @@ function collectReasoningText(part: ClientPart): string {
 
 function collectTextFromPart(part: ClientPart, t: (key: string, params?: Record<string, unknown>) => string): string {
   if (!part) return ""
+  if (isHiddenSyntheticTextPart(part)) return ""
   if (typeof (part as any).text === "string") {
     return (part as any).text as string
   }