The AgentProvider ABC modelled only the one-shot lifecycle (spawn/stop/
health_check/remove), so the interactive intake/secretary roles could never
route through a provider. Add an opt-in interactive surface:
- supports_interactive class flag (default False).
- InteractiveSpawnSpec: the resolved AgentConfig + session id + role-specific
image + optional HMAC token — everything a provider needs without importing
orchestrator internals.
- spawn_interactive(spec): a non-abstract default that declines via
ProviderError, so every existing one-shot provider is unchanged.
Pure scaffolding — no provider opts in yet (GrokProvider flips the flag when
its interactive driver lands). Zero behavioural change.
