Add age-seed-keygen CLI tool

Generates and recovers age X25519 identity keys from a BIP39 24-word
mnemonic. Uses 256 bits of entropy mapped directly to an X25519 private
key, encoded in the standard age identity file format (AGE-SECRET-KEY-1…).

Commands:
  generate  — create a new age identity and print the 24-word mnemonic
  recover   — reconstruct the exact same identity from the mnemonic

Dependencies: bech32, cryptography, mnemonic. Setup via setup.sh.

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>"