NIP-46 get_device_info method for signer metadata

[kwsantiago]

Problem

After NIP-46 register_wallet completes, keep knows the signer Nostr pubkey and an opaque HMAC token but has no over-the-wire way to learn the device kind, firmware version, or master key fingerprint. Any UI that wants to display "Coldcard Mk4" instead of a raw 32-byte pubkey has to ask the user to type it.

Solution

Extend keep-nip46 to support a get_device_info method that returns structured metadata from the signer.

{
  "kind": "Coldcard" | "Ledger" | "BitBox02" | "Jade" | "Trezor" | "Other",
  "firmware_version": "1.2.3",
  "fingerprint": "deadbeef",
  "capabilities": ["miniscript", "tapminiscript", "multisig"]
}

Reference

• keep#333: parent feature (register_wallet protocol, CLI, desktop UI)
• liana: HWI integration calls device.device_kind() and device.get_version() for the same purpose

Scope

• NIP-46 get_device_info request/response (typed, with deny_unknown_fields and size cap)
• CLI: call after connect(), populate new fields on DeviceRegistration (see companion issue)
• Desktop: populate the device kind input automatically if the signer responds
• Integration test stub in keep-nip46/tests/

Non-goals

• Implementing the server side; hardware signers implement this, we are the client
• Making get_device_info required for register_wallet to succeed (must remain best-effort)