Add OKP/Ed25519 key support to cipher

[macterra]

Problem

When issuing a credential to a did:key subject (resolved via the universal resolver fallback from #81), the cipher crashes with:

TypeError: Cannot read properties of undefined (reading 'length')

Root cause: convertJwkToCompressedBytes in packages/cipher/src/cipher-base.ts accesses jwk.y, which is undefined for Ed25519/OKP keys (they only have x). The cipher currently only supports secp256k1 EC keys.

Proposed solution

Detect OKP keys (kty === "OKP") in encryptBytes/decryptBytes and use X25519 key agreement instead of secp256k1 ECDH:

1. Convert the Ed25519 public key to X25519 using edwardsToMontgomeryPub from @noble/curves (already in the dependency tree)
2. Use x25519.getSharedSecret() for key derivation
3. The symmetric encryption (xchacha20poly1305) stays the same — only the shared secret derivation changes

Files to modify

• packages/cipher/src/cipher-base.ts — key type detection + X25519 key agreement path
• packages/cipher/src/types.ts — extend key types to support OKP JWKs
• packages/cipher/package.json — add @noble/curves as a direct dependency

Context

• Depends on Add universal resolver to gatekeeper service #81 (universal resolver) for resolving did:key DIDs
• did:key:z6Mk... prefix indicates Ed25519 keys
• Test DID: did:key:z6MkhaXgBZDvotDkL5257faiztiGiC2QtKLGpbnnEGta2doK