Busy-ack bubble repeats one canned sentence (「收到，我先记着，手头忙完再详细回你。」) #1629

Owner: Danyang Chen (@Birfy)

Follow-up to #1584 / #1586. The busy-ack bubble shown when a human message lands during a running turn is always the same fixed line — 「收到，我先记着，手头忙完再详细回你。」 / "Got it — I’ll answer properly once I finish what’s in progress." Owner feedback: 「不要死板的回复这个」.

Expected: a short, natural, one-line ack that still appears immediately, reflects the user's message (topic) and/or what the Bot is currently doing (running tool), varies its wording (never the same text twice in a row in a chat), follows the message's language, and never shows the old canned sentence verbatim.

Out of scope: #1509 (Steer does not abort a tool wait).