fix(prompt): treat terminal board entries as implicit completion signals

The background job board prompt told the orchestrator to 'Wait for
hook-driven completion', which encouraged passive waiting instead of
acting on board state changes. Changed to:
  'When a job flips to completed/reconciled on the board, treat it as
   an implicit completion notification: reconcile immediately.'

Also updated the orchestrator system prompt from 'Before final response,
reconcile any terminal jobs' to reconciling as soon as terminal jobs
appear on the board.

src/agents/orchestrator.ts

@@ -204,7 +204,7 @@ Balance: respect dependencies, avoid parallelizing what must be sequential, and
 - Continue orchestration only on non-overlapping work; otherwise briefly report what was launched and stop.
 - Before local edits or another writer task, compare against running task scopes.
 - Parallel background tasks are allowed only when their write scopes do not conflict.
-- Before final response, reconcile any terminal jobs shown in the Background Job Board.
+- Reconcile terminal jobs as soon as they appear on the Background Job Board — do not wait for hook-driven notifications before reporting results. The board is the completion signal.
 - Use \`cancel_task\` only when the user asks, or when a running lane is obsolete, wrong, or conflicts with a safer replacement plan.
 - Cancellation is not rollback: if cancelling a writer, inspect and reconcile partial file changes before launching a replacement lane.
 

src/utils/background-job-board.ts

@@ -495,7 +495,7 @@ 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.',
+        'Do not poll running jobs. When a job flips to completed/reconciled on the board, treat it as an implicit completion notification: reconcile immediately. Use cancel_task only for explicit cancellation.',
         '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.',