feat: subagent component

[arjunkmrm]

Summary

• New <Subagent> JSX component installs three tools on the parent: agent (sync), spawn_agent (background, returns id), check_agent (poll by id).
• Adds subagent.started / subagent.completed / subagent.failed events. Non-projectable; the model observes state via an ambient <subagents> block (reduced from the log) and check_agent tool results.
• Background path schedules child completion onto the parent's ManagedRuntime, which appends the terminal event directly to the parent's log via AgentCtx.events.append.

Test plan

• npx tsc --noEmit clean across src + tests (exhaustive switches in recall, halt-semantics-prop, projections-prop extended for the three new event variants).
• No new unit tests added; existing property tests cover the new event variants for projection/halt semantics.