fix(tui): drain message queue on every busy → false transition

Previously the queue only drained inside the message.complete event
handler, so anything enqueued while a shell.exec (!sleep, !cmd) or a
failed agent turn was running would stay stuck forever — neither of
those paths emits message.complete. After Ctrl+C an interrupted
session would also orphan the queue because idle() flips busy=false
locally without going through message.complete.

Single source of truth: a useEffect that watches ui.busy. When the
session is settled (sid present, busy false, not editing a queue
item), pull one message and send it. Covers agent turn end,
interrupt, shell.exec completion, error recovery, and the original
startup hydration (first-sid case) all at once.

Dropped the now-redundant dequeue/sendQueued from
createGatewayEventHandler.message.complete and the accompanying
GatewayEventHandlerContext.composer field — the effect handles it.

ui-tui/src/app/createGatewayEventHandler.ts

@@ -46,7 +46,6 @@ const pushNote = pushUnique(6)
 const pushTool = pushUnique(8)
 
 export function createGatewayEventHandler(ctx: GatewayEventHandlerContext): (ev: GatewayEvent) => void {
-  const { dequeue, queueEditRef, sendQueued } = ctx.composer
   const { rpc } = ctx.gateway
   const { STARTUP_RESUME_ID, newSession, resumeById, setCatalog } = ctx.session
   const { bellOnComplete, stdout, sys } = ctx.system
@@ -394,16 +393,6 @@ export function createGatewayEventHandler(ctx: GatewayEventHandlerContext): (ev:
           patchUiState(state => ({ ...state, usage: { ...state.usage, ...ev.payload!.usage } }))
         }
 
-        if (queueEditRef.current !== null) {
-          return
-        }
-
-        const next = dequeue()
-
-        if (next) {
-          sendQueued(next)
-        }
-
         return
       }