feat(crypto): client-side direct-message encryption with crypto_box

Direct messages between peers are now end-to-end encrypted. The
broker only ever sees {nonce, ciphertext} — plaintext lives on the
two endpoints.

apps/cli/src/crypto/envelope.ts:
- encryptDirect(message, recipientPubkeyHex, senderSecretKeyHex)
  → {nonce, ciphertext} via crypto_box_easy, 24-byte fresh nonce
- decryptDirect(envelope, senderPubkeyHex, recipientSecretKeyHex)
  → plaintext or null (null on MAC failure / malformed input)
- ed25519 keys (from Step 17) are converted to X25519 on the fly via
  crypto_sign_ed25519_{pk,sk}_to_curve25519 — one signing keypair
  covers both signing + encryption roles.

BrokerClient.send():
- if targetSpec is a 64-hex pubkey → encrypt via crypto_box
- else (broadcast "*" or channel "#foo") → base64-wrapped plaintext
  (shared-key encryption for channels lands in a later step)

InboundPush now carries:
- plaintext: string | null   (decrypted body, null if decryption failed
                              OR it's a non-direct message)
- kind: "direct" | "broadcast" | "channel" | "unknown"
MCP check_messages formatter reads plaintext directly.

side-fixes pulled in during 18a:
- apps/broker/scripts/seed-test-mesh.ts now generates real ed25519
  keypairs (the previous "aaaa…" / "bbbb…" fillers weren't valid
  curve points, so crypto_sign_ed25519_pk_to_curve25519 rejected
  them). Seed output now includes secretKey for each peer.
- apps/broker/src/broker.ts drainForMember wraps the atomic claim in
  a CTE + outer ORDER BY so FIFO ordering is SQL-sourced, not
  JS-sorted (Postgres microsecond timestamps collapse to the same
  Date.getTime() milliseconds otherwise).
- vitest.config.ts fileParallelism: false — test files share
  DB state via cleanupAllTestMeshes afterAll, so running them in
  parallel caused one file's cleanup to race another's inserts.
- integration/health.test.ts "returns 200" now uses waitFullyHealthy
  (a 200-only waiter) instead of waitHealthyOrAny — prevents a race
  with the startup DB ping.

verified live:
- apps/cli/scripts/roundtrip.ts (direct A→B): ciphertext in DB is
  opaque bytes (not base64-plaintext), decrypted correctly on arrival
- apps/cli/scripts/join-roundtrip.ts (full join → encrypted send):
  PASSED
- 48/48 broker tests green

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

apps/cli/src/mcp/server.ts

@@ -74,13 +74,7 @@ function resolveClient(to: string): {
 }
 
 function formatPush(p: InboundPush, meshSlug: string): string {
-  const body = (() => {
-    try {
-      return Buffer.from(p.ciphertext, "base64").toString("utf-8");
-    } catch {
-      return "(invalid base64 ciphertext)";
-    }
-  })();
+  const body = p.plaintext ?? "(decryption failed)";
   return `[${meshSlug}] from ${p.senderPubkey.slice(0, 12)}… (${p.priority}, ${p.createdAt}):\n${body}`;
 }