chat: a queued message is pasted over a draft already in the agent's input #609

Reported by the merge lane for #601 (lane ext-601) while reviewing the pending-input path; pre-existing on main, not introduced by #601. Read from the code by the lane; not reproduced by the lead.

A queued chat message is delivered by pasting into the agent's input and pressing Enter (pendingContext and the delivery loop in server/index.ts). If the agent's input box already holds a draft the user typed in the terminal, the paste lands on top of it and the Enter sends both together. After #601 the same applies to a message Claude Code holds back for invisible characters once the ten-minute note of it has expired (HELD_WINDOW_MS in server/prompt.ts), or once it was edited in the terminal.

Expected: delivery waits (or refuses with a clear reason) while the input box is not empty, the way it already waits behind a prompt card. For Claude Code the box can be read from the screen (the ❯ line between the two rules; grey placeholder text is not a draft, see claudeGreyInput). Other agents need their own reader, or the rule stays Claude-only at first.