fix(review): sum live tokens across a collapsed agent's workers

Follow-on to the one-row-per-agent collapse: #1666 (merged) streams cumulative token counts per worker, and the TUI row overwrites on each Tokens event (CU2). With two skill-workers folded into one agent row, their independent cumulative counts overwrote each other — the live number bounced between skills instead of showing the agent total. (The final summary was already summed; only the live display was wrong.)

The sink now tracks each worker's latest cumulative Tokens and forwards the per-agent SUM on every Tokens event, so the row's overwrite shows a correct running total. Single-worker rows pass through unchanged. Runs on the serial dispatch goroutine, so no locking; race-clean.