Wire CLI wallet propose to Nostr descriptor session

[kwsantiago]

The descriptor coordination protocol is implemented in keep-frost-net but the CLI wallet propose command doesn't exist yet. Need to:

• Add keep wallet propose --group <hex> --network <net> --recovery "2of3@6mo" CLI command
• Connect it to KfpNode::request_descriptor()
• Listen for DescriptorContributionNeeded / DescriptorComplete events
• Store the finalized descriptor via keep.store_wallet_descriptor()

The protocol layer, session management, and storage are all done — this is the CLI integration glue.