Auto-shield mining rewards + atomic-swap CLI

Two features in one commit: 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. Atomic-swap CLI — zer-cli swap btc:0.001 zer:200 opens an HTLC against a remote counterparty,…