frost network sign-event: hardware path requires aggregate-receive plumbing

[kwsantiago]

Follow-up to #497 (PR landing now).

#497 ships software `keep frost network sign-event`. The hardware path is rejected with `NotImplemented`:

```
hardware FROST network sign-event requires aggregate-receive plumbing on the hardware path;
run the same group with a software participant who can aggregate, or see follow-up issue.
```

Why

`cmd_frost_network_sign_hardware` (keep-cli/src/commands/frost_network/hardware.rs) publishes the local signature share to the relay and exits with "Waiting for coordinator to aggregate final signature". The 64-byte aggregated schnorr signature is never observed locally, so we cannot assemble the signed Nostr event on this side. `sign-event` requires the full sig to build the JSON `Event`.

Scope

1. Define the wire protocol for a hardware participant to receive the aggregate from the coordinator (probably a new `kind = 21107` event tagged with the request_id, content = hex sig).
2. Have `cmd_frost_network_sign_hardware` subscribe to that filter after publishing its share and wait (with timeout) for the aggregate.
3. Add a parallel hardware sign-event entry point that reuses the same waiter and builds the signed event JSON.
4. Tests: covers the wait-for-aggregate path, including timeout when the coordinator never publishes.

Related

• Implement frost network sign-event end-to-end (signed Nostr event output) #497 / PR (TBD): software sign-event shipped here.
• `keep-cli/src/commands/frost_network/hardware.rs` (current hardware sign).