orchestrate: adopt Trinity ephemeral (ghost) agents for disposable workers — replace durable eph-* deploy + best-effort teardown

[vybe]

Summary

The /orchestrate skill (agent-dev /add-orchestrator bundle) provisions disposable workers as durable agents named eph-<agent>-<short-id> via deploy_system, then tears them down best-effort with stop_agent + delete_agent — and deliberately skips teardown on failed runs, so a failed or interrupted orchestration leaves containers behind with no GC.

Trinity now ships first-class ephemeral agents (Abilityai/trinity#1580): create_agent accepts ephemeral: true plus a hard budget (max_executions and/or ttl_seconds), the platform hard-discards the agent at budget (container, DB rows, name — no soft-delete/retention), and a cleanup sweep reclaims orphans even after crashes. Spawn provenance auto-grants the parent→child permission edge, so the orchestrator can immediately chat with / control the child it spawned.

Acceptance Criteria

• /orchestrate creates workers via create_agent with ephemeral: true and an explicit budget (max_executions sized to the planned runs, or a ttl_seconds window) instead of the durable eph-* + deploy_system path.
• Parent control relies on the auto-granted spawn permission edge — no manual permission-grant step in the skill.
• Teardown logic simplified: platform GC is the safety net; early discard via delete_agent when a worker finishes ahead of budget. No more skip-teardown-on-failure leak.
• Graceful fallback: on 403 ephemeral_not_entitled (instance without the entitlement), fall back to the current durable eph-* flow with a one-time warning — the skill must keep working on OSS instances.
• Failed-run inspection story documented: ghost execution rows are kept on discard and remain queryable via execution history (until standard retention); for interactive debugging, recommend a longer TTL rather than skipping teardown.
• Plugin README / skill docs updated.

Technical Notes

• Ephemeral creation intentionally skips credential auto-injection and defaults max_parallel_tasks=1; per-owner quota (default 5 live ghosts) and a per-parent spawn rate limit apply — the skill should surface a clear message when quota-limited rather than retry-looping.
• An ephemeral agent cannot spawn another ephemeral agent (depth-1 by design) — orchestrator must be a durable agent.