test(cli): add crypto roundtrip and invite parse tests

Cover encryptDirect/decryptDirect with three scenarios (happy path,
wrong recipient, tampered ciphertext) and invite link parsing with
round-trip, expiry rejection, and malformed input handling.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/cli/src/__tests__/crypto-roundtrip.test.ts

@@ -0,0 +1,42 @@
+import { describe, it, expect } from "vitest";
+import { encryptDirect, decryptDirect } from "../crypto/envelope";
+import { generateKeypair } from "../crypto/keypair";
+
+describe("crypto roundtrip", () => {
+  it("Alice encrypts for Bob, Bob decrypts successfully", async () => {
+    const alice = await generateKeypair();
+    const bob = await generateKeypair();
+
+    const plaintext = "hello world";
+    const envelope = await encryptDirect(plaintext, bob.publicKey, alice.secretKey);
+
+    const decrypted = await decryptDirect(envelope, alice.publicKey, bob.secretKey);
+    expect(decrypted).toBe(plaintext);
+  });
+
+  it("Carol cannot decrypt a message encrypted for Bob", async () => {
+    const alice = await generateKeypair();
+    const bob = await generateKeypair();
+    const carol = await generateKeypair();
+
+    const envelope = await encryptDirect("hello world", bob.publicKey, alice.secretKey);
+
+    const decrypted = await decryptDirect(envelope, alice.publicKey, carol.secretKey);
+    expect(decrypted).toBeNull();
+  });
+
+  it("tampered ciphertext returns null on decrypt", async () => {
+    const alice = await generateKeypair();
+    const bob = await generateKeypair();
+
+    const envelope = await encryptDirect("hello world", bob.publicKey, alice.secretKey);
+
+    // Flip a byte in the ciphertext
+    const raw = Buffer.from(envelope.ciphertext, "base64");
+    raw[0] = raw[0]! ^ 0xff;
+    const tampered = { nonce: envelope.nonce, ciphertext: raw.toString("base64") };
+
+    const decrypted = await decryptDirect(tampered, alice.publicKey, bob.secretKey);
+    expect(decrypted).toBeNull();
+  });
+});