refactor(acp): simplify the turn fold - #52826

-ACPTranslate.step(state, event, ctx: Context): Step
-  Context.mode: "turn" | "background"
-  if background and not a child: drop event
-  route(): drop SessionUpdate when background
-  FormAsk.toolCallSent = mode === "turn" && ...
-  switch (event.type)
-    step.started / step.ended / step.failed:  child ? ... : ...
-    execution.succeeded / interrupted / failed: child ? childEnded(..., terminal) : terminal
+ACPTranslate.fold(state, event, ctx: TurnContext): Folded
+  session.created            -> childCreated
+  asks and settled asks      -> shared
+  owner = tracked child ?? root session
+  child ? childEvent(...) : rootEvent(...)
+    rootEvent:  step usage, step error, terminal
+    childEvent: child status, child ended (no terminal)
+    both fall back to sessionEvent: retries, compaction, text, reasoning, tools
+ACPTranslate.fromTrackedChild(state, event)   # filter for the background follow

-type Subscription = { scope, events, asks, cancelled, settled }
+type Subscription = { scope, events, askQueue, settled }
+type Turn = { ctx, state, subscription, cancelled, background }

-reply(subscription, ctx, ask, settled)
-interpret(subscription, ctx, output)
-consume(subscription, ctx, state)
-windDown(subscription, ctx, state, events)
-execute(attached, prompt, ctx, state)
-handoff(attached, subscription, ctx, state, exit)
-settle(attached, state: Ref, exit)
+queueAsk(turn, id, ask, fallback, respond)   # races cancel, settled, then ask; responds uninterruptibly
+interpret(turn, output)          # drops SessionUpdate and computes toolCallSent when background
+advance(turn)                    # take, filter with fromTrackedChild when background, fold, interpret
+consume(turn)                    # ends on the root terminal
+followChildren(turn)             # ends when openChildren is empty
+windDown(turn, events)
+execute(attached, prompt, turn)
+handoff(attached, turn, exit)
+settle(attached, state: TurnState, exit)

-ACPPermission.reply(input & { settled }, cancelled)
-ACPElicitation.reply(input & { settled }, cancelled)
+ACPPermission.ask(input)      ACPPermission.respond(input, decision)
+ACPElicitation.ask(input)     ACPElicitation.respond(input, outcome)

 packages/cli/src/acp/
+├── prompt.ts      # ACPPrompt.prepare: image check, unreadable file links, slash and compaction detection
 ├── permission.ts  # exports ask and respond
 ├── elicitation.ts # exports ask and respond
 ├── tool.ts        # canonicalName replaces six lowercase alias switches
 ├── translate.ts   # fold, root and child handlers
 └── turn.ts        # Turn record, ask race, delivery policy, background follow