Add receipt keygen and receipt verify commands

[KryptosAI]

What

Complete the Ed25519 receipt signing loop with two new subcommands on mcp-observatory receipt:

receipt keygen

Generates an Ed25519 key pair and writes them to files (or prints to stdout):

$ mcp-observatory receipt keygen
Public key saved:  mcp-observatory.pub
Private key saved: mcp-observatory.key

Keep the private key secure. Share the public key with anyone who needs to verify your receipts.

receipt verify <file> --key <pubkey>

Verifies a signed receipt against a public key:

$ mcp-observatory receipt verify receipt.json --key mcp-observatory.pub
✓ Receipt verified — signed by maintainer@example.com

Files

• src/commands/receipt.ts — add two new subcommands
• src/receipt.ts — verifyReceipt() and generateReceiptKeyPair() already exist

Acceptance

• receipt keygen writes DER-encoded key files
• receipt verify confirms signature, shows signer identity
• Tests for both commands
• npx tsc --noEmit clean