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.

README.md

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+# age-seed-keygen
+
+An age identity generator with a BIP39 recovery phrase. Every key comes with 24 words you can write down — lose the file, say the words, get it back.
+
+## The idea
+
+age X25519 identity keys are 32 random bytes. BIP39 is a standard for encoding random bytes as human-readable words (the same standard hardware wallets use). This tool generates 256 bits of entropy, turns it into both an age identity and a 24-word mnemonic, and gives you both. Recovery is the reverse — give back the 24 words, get back the exact same identity.
+
+If you already use `ssh-seed-keygen`, you can back up both an SSH key and an age identity from a single mnemonic — or keep them separate. Either way, one piece of paper is all you need.
+
+## Getting started
+
+```bash
+bash setup.sh
+```
+
+Creates a virtualenv and installs the three dependencies.
+
+## Generating an identity
+
+```bash
+.venv/bin/python keygen.py generate
+```
+
+Writes the identity file to `~/.config/age/key.txt` by default, then prints your 24 words. Write them down somewhere offline.
+
+```bash
+# different output path
+.venv/bin/python keygen.py generate -o ~/my-age-key.txt
+```
+
+## Recovering an identity
+
+```bash
+# pass the words directly
+.venv/bin/python keygen.py recover word1 word2 ... word24
+
+# or run it and paste when prompted
+.venv/bin/python keygen.py recover
+```
+
+Same `-o` flag applies if you want the recovered file somewhere other than the default path.
+
+## Using the identity
+
+The output file is a standard age identity file — it works directly with the `age` CLI:
+
+```bash
+# encrypt a file
+age -r age1<your-public-key> secret.txt > secret.txt.age
+
+# decrypt using the identity file
+age --decrypt -i ~/.config/age/key.txt secret.txt.age > secret.txt
+```
+
+## Protecting the identity file
+
+The identity file is written with mode `0600`. If you want to encrypt it at rest, use age itself:
+
+```bash
+age --passphrase -o key.txt.age ~/.config/age/key.txt
+rm ~/.config/age/key.txt
+```
+
+Decrypt before use:
+
+```bash
+age --decrypt key.txt.age > ~/.config/age/key.txt
+```
+
+## One thing to keep in mind
+
+The mnemonic encodes the private key directly. Anyone with those 24 words has your identity. Treat them at least as carefully as the key file itself.