Auto-shield mining rewards + atomic-swap CLI

Two features in one commit:

1. Auto-shield mining rewards — when a Vanta block matures (30 confirmations), the wallet automatically converts the coinbase into a shielded note. No manual shield 6.25 step. The miner who runs the wallet is now indistinguishable on-chain from any other shielded participant.
2. Atomic-swap CLI — zer-cli swap btc:0.001 zer:200 opens an HTLC against a remote counterparty, watches both chains for confirmations, and refunds on timeout. First production-quality BTC ↔ ZER UX.

The auto-shield is the one users notice. Half the value of a privacy chain is everybody using the privacy features by default.