fix(job-board): remove misleading 'do not poll' instruction from prompt

The Background Job Board prompt text instructed the orchestrator to avoid
polling running jobs, but the agent has no poll/status tool — completion is
hook-driven. The line was inaccurate and has been removed. The golden
snapshot was updated deliberately to reflect the trimmed board text.

src/hooks/__snapshots__/cache-payload.snapshot.test.ts.snap

@@ -467,7 +467,6 @@ exports[`cache-impact snapshots (update deliberately — see file header) transf
 "<system-reminder>
 ### Background Job Board
 SENTINEL: background-job-board-v2
-Do not poll running jobs. Wait for hook-driven completion, or use cancel_task only for explicit cancellation. Reconcile terminal jobs before final response.
 Completed or reconciled sessions are reusable by alias for the same specialist/context.
 Timed-out running sessions are recoverable by alias for safe resume after a live busy signal.
 Cancelled or errored sessions are not reusable.

src/utils/background-job-board.ts

@@ -495,7 +495,6 @@ export class BackgroundJobBoard implements BackgroundJobStore {
       [
         '### Background Job Board',
         'SENTINEL: background-job-board-v2',
-        'Do not poll running jobs. Wait for hook-driven completion, or use cancel_task only for explicit cancellation. Reconcile terminal jobs before final response.',
         'Completed or reconciled sessions are reusable by alias for the same specialist/context.',
         'Timed-out running sessions are recoverable by alias for safe resume after a live busy signal.',
         'Cancelled or errored sessions are not reusable.',