fix(scheduler): dispatch executor-excluded handoffs instead of repolling origin monitor #4311

问题 / Problem

当一个 continuous monitor 在 material transition 后创建 continuation_policy=independent_handoff 的 advancement Todo，并用 excluded_agents=[origin-agent] 禁止原作者自审时，Todo 会保持 unclaimed；但后续 heartbeat 仍持续唤醒 origin agent 轮询同一 monitor。即使 consecutive_no_change >= 2，当前行为也只建议调大轮询间隔，不会把真实可推进的 handoff 分派给同 Goal 的另一个 Agent。

When a continuous monitor emits an advancement Todo with continuation_policy=independent_handoff and excludes the origin agent, the Todo can remain unclaimed while later heartbeats repeatedly select the origin monitor. Even after multiple unchanged polls, the runtime only proposes polling backoff; it does not dispatch the real handoff to another agent in the same Goal.

可复现证据 / Reproduction evidence

1. Origin agent owns a due continuous_monitor.
2. A material transition creates one linked advancement Todo with excluded_agents=[origin-agent].
3. The projected priority list shows that unclaimed Todo, while the current-agent claim scope reports zero selectable unclaimed work.
4. manager.context_inbox is empty at turn start because no durable dispatch/receipt was produced for another agent.
5. The origin agent receives repeated monitor turns; unchanged polls increase consecutive_no_change, but no peer claim or handoff occurs.

This is not a missing-work problem: actionable work exists, but the scheduler/claim/handoff bridge cannot route it to an eligible executor.

根因划分 / Root-cause split

• Agent-side: the origin agent must not equate a successful poll with whole-goal progress; it should verify that an independent successor has a receiver and return receipt.
• LoopX-side: the control plane lacks an idempotent broker from an unclaimed, executor-excluded successor to an eligible peer/session. Current projection also conflates “unclaimed globally” with “selectable by this agent”, hiding dispatch starvation.

期望 / Expected behavior

• A material monitor transition atomically links the successor and emits a durable dispatch obligation.
• Turn-start handoff/inbox logic claims the successor exactly once for a different registered agent in the same Goal, or projects an explicit no_eligible_peer blocker.
• The origin monitor may continue at its configured cadence, but it must not starve independent advancement work or force a second automation/source of truth.
• Codex message delivery alone is not acceptance; dispatch requires inbox visibility plus a return receipt.

验收 / Acceptance

• Frontier projection distinguishes dispatchable_by_peer from unclaimed_but_executor_excluded.
• agent_handoff -> turn_start hook -> inbox read -> return receipt survives restart and is idempotent under duplicate delivery.
• After the configured unchanged threshold, origin-agent routing yields to a dispatch/replan action instead of only suggesting interval backoff.
• If no eligible peer exists, a typed blocker is projected without inventing a user gate.
• CLI/managed Turn, frontend, and Lark read the same typed dispatch/receipt projection.
• Tests cover duplicate heartbeats, stale claims, restart, and same-Goal agent identity isolation.