feat(bg): B-0500 slice 3 — wire queue-state guard into pollOnce

[AceHack]

Implements backlog row B-0500 (Slice 3 of the B-0441 notifier). We now properly guard work assignments using isAgentQueueEmpty, ensuring agents only receive new proactive tasks when their queues are fully cleared.

[Copilot]

Pull request overview

This PR completes Slice 3 of the B-0441 backlog-ready notifier by wiring the isAgentQueueEmpty queue-state guard into pollOnce. When the target agent's queue is non-empty (recent commits or open PRs by that agent), the poll iteration returns early with queueBusy: true and publishes no work-assignment envelopes, preventing assignment pile-up on busy agents.

Changes:

• Add targetAgent field to NotifierConfig and a queueBusy field to PollResult; populate via isAgentQueueEmpty(config.targetAgent, adapters) at the top of pollOnce.
• Thread adapters through runOnce/runDaemon and add a --target-agent <agent> CLI flag.
• Add two new tests (queue-busy → no publish; queue-empty → publishes) and update backlog rows B-0500/B-0441 to closed/shipped.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
tools/bg/backlog-ready-notifier.ts	Wire queue-state guard into pollOnce; add targetAgent config + queueBusy result field; plumb adapters through runOnce/runDaemon; add --target-agent flag.
tools/bg/backlog-ready-notifier.test.ts	Add tests for queue-busy skip and queue-empty publish paths; update existing tests for new config field/flag.
docs/backlog/P1/B-0500-...md	Mark B-0500 status closed and acceptance checkboxes ticked.
docs/backlog/P1/B-0441-...md	Update slice-3 row of B-0441 table to ✅ shipped.