Implements docs/phase2.md §5 (Sub-agents), pulled forward from the
harness's item 2 work: SubAgent/SubAgentRegistry let a caller run a
nested agent.Loop with its own persona/tools/iteration cap and get its
final response back. Run doesn't set Approver/Sandbox, so a single Ask
approval on the caller's own delegating tool covers the whole nested
run (Ask-permission tools execute unprompted when Config.Approver is
nil). rony-harness consumes this for its delegate tool (builder/planner).
