Add SSH key generator with BIP39 mnemonic backup

Generates Ed25519 SSH keys and encodes the private key seed as 24 BIP39
words for safe offline storage. Keys can be fully recovered from the
mnemonic alone.

setup.sh

@@ -0,0 +1,6 @@
+#!/usr/bin/env bash
+# One-time setup: creates a venv and installs dependencies
+set -e
+python3 -m venv .venv
+.venv/bin/pip install -q -r requirements.txt
+echo "Setup complete. Run with: .venv/bin/python keygen.py <command>"