[P2] Define busy-thread delivery and evaluate Codex’s native queue #38

Summary: Decide behavior for busy destinations and investigate native queue APIs before custom queueing.
Why: only resume then turn/start exists; active-turn and admission-race semantics are untested; v0.154.0 exposes experimental queue methods but durability/compatibility are unknown.
Acceptance criteria: source/experiment-backed decision; test persistence, ordering, restart, activation, active-turn races, and idle/active/external/unsupported destinations; gate experimental APIs; distinguish queued/started/completed; never steer or close human work.

Parent: #24
Source: GPT-6 Pro post-land review of #27/#28 — https://chatgpt.com/c/6aa89f23-6920-83ea-b98a-a1f155a1ae3f