Run/RunStream took the new turn as a bare string, which had nowhere
to carry ContentPart attachments. Both now take an llm.Message
(Role is forced to RoleUser regardless of what the caller sets), so a
caller building a multimodal turn just fills in Content/Parts on it
instead of the loop needing a second, parallel parameter.
subagent.go and every test call site are updated to wrap their string
prompt as llm.Message{Role: llm.RoleUser, Content: ...} — SubAgent.Run
itself is untouched, it still takes a plain task string.
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).
