feat: add export and post-quantum signing

[akeemjenkins]

Summary

• okf export — Export an entire OKF bundle as a deterministic .okf tar.gz archive. The archive is byte-reproducible (sorted paths, zeroed timestamps) so the same bundle always produces the same bytes. Output includes a manifest with per-file SHA-256 hashes, file count, total bytes, and archive hash.

• okf sign — Post-quantum authentication for exported archives using ML-KEM-768 (FIPS 203, formerly Kyber) via HPKE (RFC 9180). Uses only the Go standard library (crypto/hpke + crypto/mlkem) — no external dependencies.

  • keygen — generate an ML-KEM-768 key pair
  • sign — seal the archive hash with HPKE using the public key
  • verify — open the HPKE ciphertext with the private key and confirm the archive hash matches (detects tampering)

Both commands are registered in the schema (okf schema) for agentic-first discovery.

How it works

ML-KEM is a key-encapsulation mechanism (KEM), not a digital signature scheme. This feature uses HPKE to achieve authenticated encryption of the archive hash:

1. Sign: hpke.Seal(pubkey, hash) produces a ciphertext containing the encapsulated key + sealed archive hash
2. Verify: hpke.Open(privkey, ciphertext) recovers the archive hash; if it doesn't match the actual archive hash, the archive has been tampered with

Only the holder of the corresponding private key can open the ciphertext, proving the archive was sealed for that key and has not been modified.

Test plan

• 5 export tests: archive creation, determinism, hidden dir skipping, non-directory error, manifest file hashes
• 4 sign tests: key generation, sign+verify happy path, tampered archive detection, wrong key rejection
• Full test suite passes (44 tests, race-enabled)
• go vet clean
• End-to-end: export → keygen → sign → verify cycle verified

okf export ./testdata/valid -o bundle.okf
okf sign bundle.okf keygen
okf sign bundle.okf sign --pub <pubkey> -o sig.json
okf sign bundle.okf verify --priv <privkey> --sig sig.json
# { "verified": true }