refine signing example review fixes

Author: Copilot

.agents/skills/building-with-hypercerts-lexicons/SKILL.md

@@ -557,6 +557,9 @@ const cidBytes = cid.bytes; // 36 bytes: 0x01 0x71 0x12 0x20 + 32-byte hash
 //    ECDSA hash-then-sign convention and enforces low-S per BIP-0062.
 const keypair = await Secp256k1Keypair.create({ exportable: false });
 const signerDid = keypair.did();
+if (!signerDid.startsWith("did:key:")) {
+  throw new Error(`Expected did:key signer, got ${signerDid}`);
+}
 const signerKey = `${signerDid}#${signerDid.slice("did:key:".length)}`;
 const signatureBytes = await keypair.sign(cidBytes);
 

README.md

@@ -641,6 +641,9 @@ const cidBytes = cid.bytes; // 36 bytes: 0x01 0x71 0x12 0x20 + 32-byte hash
 //    ECDSA hash-then-sign convention and enforces low-S per BIP-0062.
 const keypair = await Secp256k1Keypair.create({ exportable: false });
 const signerDid = keypair.did();
+if (!signerDid.startsWith("did:key:")) {
+  throw new Error(`Expected did:key signer, got ${signerDid}`);
+}
 const signerKey = `${signerDid}#${signerDid.slice("did:key:".length)}`;
 const signatureBytes = await keypair.sign(cidBytes);
 

tests/validate-signing-example.test.ts

@@ -26,7 +26,7 @@ import { verifySignature } from "@atproto/crypto";
 import * as Activity from "../generated/types/org/hypercerts/claim/activity.js";
 
 const ROOT = resolve(dirname(fileURLToPath(import.meta.url)), "..");
-const EXTRACT_DIR = resolve(ROOT, "tmp/extracted-signing-examples");
+const EXTRACT_DIR = resolve(ROOT, "tmp/extracted");
 
 interface ExtractedExample {
   signedActivity: { signatures: unknown[] };
@@ -244,7 +244,7 @@ for (const [label, relPath, basename] of SOURCES) {
       expect(result.success).toBe(true);
     });
 
-    it("produced signature verifies against the signing public key", async () => {
+    it("the produced signature verifies against the signing public key", async () => {
       const ok = await verifySignature(
         example.keypair.did(),
         example.cidBytes,