fix(sdk): surface nested interrupts on stream.interrupts (#2672)

Nested `interrupt()` calls (subgraphs / createDeepAgent subagents) never
showed up on `useStream().interrupts` while the run was live —
`#recordRootInterrupt` early-returned on non-root namespaces — but
hydrate seeded them from `state.tasks`, so the same interrupt appeared
after a reload. Mirror every `input.requested` onto
`rootStore.interrupts` with `Interrupt.namespace`, and look that
namespace up in `respond({ interruptId })` when it isn't passed
explicitly.