feat: add fuzz testing harness and fix repost builder

- Created generic fuzzing library `scripts/agent/fuzz-lib.mjs`.
- Implemented fuzz targets for `nostrEventSchemas.js`, `dmDecryptor.js`, and `magnetUtils.js`.
- Fixed `buildRepostEvent` to allow missing relay URLs for robustness.
- Added crash reproducers to `examples/reproducers/`.

scripts/agent/fuzz-dm-decryptor.mjs

@@ -0,0 +1,72 @@
+import { Fuzzer } from "./fuzz-lib.mjs";
+import { decryptDM } from "../../js/dmDecryptor.js";
+
+// Mock global dependencies
+if (typeof global.window === "undefined") {
+    global.window = {
+        location: { protocol: "https:" },
+        crypto: {
+             getRandomValues: (arr) => crypto.randomFillSync(arr),
+             subtle: {
+                 digest: async (algo, data) => {
+                     const hash = crypto.createHash("sha256").update(data).digest();
+                     return hash.buffer;
+                 }
+             }
+        }
+    };
+}
+if (typeof global.localStorage === "undefined") {
+    global.localStorage = {
+        getItem: () => null,
+        setItem: () => {},
+        removeItem: () => {}
+    };
+}
+
+const fuzzer = new Fuzzer("dm-decryptor");
+
+async function test(fuzzer, state) {
+    // Generate random event
+    const event = {
+        kind: fuzzer.randBool() ? fuzzer.pick([4, 1059]) : fuzzer.randInt(0, 20000), // 4=DM, 1059=GiftWrap
+        created_at: Math.floor(Date.now() / 1000),
+        tags: fuzzer.randArray(() => {
+             const tagType = fuzzer.pick(["p", "e", "encrypted"]);
+             if (tagType === "p") return ["p", fuzzer.randString(64, "0123456789abcdef")];
+             if (tagType === "encrypted") return ["encrypted", fuzzer.pick(["nip04", "nip44", "nip44_v2"])];
+             return [fuzzer.randString(5), fuzzer.randString(10)];
+        }, 0, 5),
+        content: fuzzer.randUnicodeString(fuzzer.randInt(0, 200)),
+        pubkey: fuzzer.randString(64, "0123456789abcdef"),
+        id: fuzzer.randString(64, "0123456789abcdef")
+    };
+
+    if (fuzzer.randBool()) event.content = fuzzer.randJSON(); // Garbage content
+
+    // Mock decryptors
+    const decryptors = fuzzer.randArray(() => {
+        return {
+            decrypt: async (pubkey, ciphertext) => {
+                if (fuzzer.randBool()) throw new Error("Decrypt failed");
+                // Return random valid JSON or garbage
+                if (fuzzer.randBool()) return JSON.stringify(fuzzer.randJSON());
+                return fuzzer.randUnicodeString(50);
+            },
+            scheme: fuzzer.pick(["nip04", "nip44", "nip44_v2", "unknown"]),
+            priority: fuzzer.randInt(0, 10),
+            supportsGiftWrap: fuzzer.randBool()
+        };
+    }, 0, 3);
+
+    const context = {
+        actorPubkey: fuzzer.randString(64, "0123456789abcdef"),
+        decryptors: decryptors
+    };
+
+    state.input = { event, context };
+
+    await decryptDM(event, context);
+}
+
+fuzzer.runFuzzLoop(2000, test);