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-magnet-utils.mjs

@@ -0,0 +1,36 @@
+import { Fuzzer } from "./fuzz-lib.mjs";
+import { normalizeAndAugmentMagnet } from "../../js/magnetUtils.js";
+
+// Mock global dependencies if needed
+if (typeof global.window === "undefined") {
+    global.window = {
+        location: { protocol: "https:" }
+    };
+}
+
+const fuzzer = new Fuzzer("magnet-utils");
+
+async function test(fuzzer, state) {
+    const rawValue = fuzzer.randBool() ?
+        `magnet:?xt=urn:btih:${fuzzer.randString(40, "0123456789abcdef")}&dn=${fuzzer.randString(10)}` :
+        fuzzer.randUnicodeString(100);
+
+    const options = {
+        webSeed: fuzzer.randBool() ? fuzzer.randString(50) : fuzzer.randArray(() => fuzzer.randString(50), 0, 3),
+        torrentUrl: fuzzer.randBool() ? fuzzer.randString(50) : undefined,
+        xs: fuzzer.randBool() ? fuzzer.randString(50) : undefined,
+        extraTrackers: fuzzer.randArray(() => fuzzer.randString(30), 0, 5),
+        appProtocol: fuzzer.pick(["http:", "https:", "magnet:", "unknown:"]),
+        logger: () => {} // No-op logger
+    };
+
+    // Sometimes pass garbage options
+    if (fuzzer.randBool()) {
+        options.webSeed = fuzzer.randJSON();
+    }
+
+    state.input = { rawValue, options };
+    normalizeAndAugmentMagnet(rawValue, options);
+}
+
+fuzzer.runFuzzLoop(5000, test);