tasks/orchestration: completion delivery loop, CompletionRouter registry, abort guard, live-then-durable wait/resolve #473

Generic detached-run plumbing that OpenHuman reimplements in agent/orchestration/ (see openhuman#7929):

• completions::delivery in tinyagents-tasks. A debounced, batch-claiming loop with backoff, give-up and boot recovery, plus a parent-busy RAII gate, a one-turn-per-thread lease and drain timing.
  • Seams: is_busy, the delivery turn, the undelivered writer, and a fence trait.
  • From background_delivery.rs (749), busy_guard.rs, background_delivery/slot.rs and delivery_drain.rs.
• CompletionRouterPool<K>. A lazily opened router per key with in-memory fallback, mirroring TaskStoreRegistry. From background_completions.rs.
• Watcher writes the run ledger. spawn_status_watcher settles AgentRunStatus itself (from run_ledger_finalize.rs).
• DetachedTaskRegistry abort guard. Emit a terminal event when take_and_cancel drops the future (from tools/subagent_abort_report.rs).
• detached::*_or_durable combinators. Try the live registry, then the TaskStore, for wait / resolve / cancel (from running_subagents/{wait,resolve,cancel}.rs). Also return metadata for cancelled runs.
• wait_all. Multi-child deadline wait (from orchestration/ops.rs).
• Decide awaiting_input handling in DetachedCompletionTarget (see Background completions record awaiting_input as a completion, taking the dedupe slot openhuman#7932).

Done when: merged upstream with tests in sibling *_tests.rs, released or tagged, and the OpenHuman follow-up can delete the host copy after a gitlink bump on vendor/tinyagents.

Parent: tinyhumansai/openhuman#7929, which has the full file-by-file verdict tables.