feat: add export and post-quantum signing (ML-DSA-65)

[akeemjenkins]

Summary

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

• okf sign provides post-quantum digital signatures for exported archives using ML-DSA-65 (FIPS 204, formerly CRYSTALS-Dilithium) via cloudflare/circl.

  • keygen generates an ML-DSA-65 key pair (private key stored as the 32-byte FIPS 204 seed)
  • sign signs the archive's SHA-256 hash with the private key
  • verify checks the signature with the signer's public key and confirms the archive is untampered

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

Design note

An earlier revision of this branch used ML-KEM-768 + HPKE. ML-KEM is a key-encapsulation mechanism, not a signature scheme: anyone holding the public key could seal a fresh "signature" over a tampered archive, and verification required the private key. That revision was replaced with ML-DSA-65, the NIST post-quantum signature standard, with the conventional key roles: private key signs, public key verifies. Verification always uses the public key supplied on the command line, never the one embedded in the signature file.

Test plan

• 5 export tests: archive creation, determinism, hidden dir skipping, non-directory error, manifest file hashes
• 6 sign tests: key generation, sign+verify happy path, tampered archive, wrong key, forged signature (public key cannot sign; corrupted signature rejected), wrong algorithm
• Full test suite passes with -race, go vet and golangci-lint clean on PR packages
• End-to-end: export twice produces identical bytes; export -> keygen -> sign -> verify cycle passes; tampered archive fails verification with a structured error; signing with the public key is rejected

okf export ./testdata/valid -o bundle.okf
okf sign bundle.okf keygen
okf sign bundle.okf sign --priv <private-key-hex> -o sig.json
okf sign bundle.okf verify --pub <public-key-hex> --sig sig.json
# { "verified": true, "algorithm": "ML-DSA-65" }

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