Adds `uwf step ask <step-hash> -p <prompt>` for asking follow-up
questions to a completed step's agent without mutating thread state.
- Fork-by-default: creates and caches a fork session per step (cache
key `<stepHash>:ask`); subsequent asks reuse it.
- `--no-fork` fallback: spawns a fresh session with the step's detail
ref injected as context.
- `--agent` overrides the recorded agent; otherwise resolves from the
step's agent field via config alias.
- Updates `packages/cli/README.md` and `packages/util/src/usage-reference.ts`
so the new subcommand is discoverable via README and `uwf prompt usage`.
Fixes#146
