[Fix] Brain backfills delay recent task context

[mrubens]

​Opened on behalf of @mrubens. View the task or mention @roomote for follow-up asks.

What changed

• Persist each task run's completion time on its Brain memory outbox event.
• Claim active events through a partial index ordered by completion time and descending run ID, while preserving atomic FOR UPDATE SKIP LOCKED claims and stale-processing recovery.
• Backfill priority timestamps for existing outbox rows during migration and populate them through completion, agent-summary, and historical-backfill enqueue paths.
• Cover newest-first ordering, deterministic ties, and timestamp propagation with real-database tests.

Why this change was made

Historical events can share effectively identical outbox creation times. Joining and sorting the full pending backlog for every small claim made recent context expensive to prioritize during a large Brain backfill.

Impact

Recent completed tasks become available to Brain first without repeatedly joining and sorting the historical backlog. The queue keeps its existing concurrency and retry behavior, and the migration safely initializes priority metadata for events already waiting to drain.

[roomote-community]

No code issues found. See task

• packages/db/src/lib/brain.ts - Claims use persisted completion priority and a matching active-queue index instead of joining and sorting the full pending backfill.

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