feat(broker): verify ed25519 hello signature against member pubkey

WS handshake is now authenticated end-to-end. The broker proves that
every connected peer actually holds the secret key for the pubkey
they claim as identity — not just that they know the pubkey.

wire format change:
  {type:"hello", meshId, memberId, pubkey, sessionId, pid, cwd,
   timestamp, signature}
  where signature = ed25519_sign(canonical, secretKey)
  and canonical = `${meshId}|${memberId}|${pubkey}|${timestamp}`

broker verifies on every hello:
1. timestamp within ±60s of broker clock → else close(1008, timestamp_skew)
2. pubkey is 64 hex chars, signature is 128 hex chars → else malformed
3. crypto_sign_verify_detached(signature, canonical, pubkey) → else bad_signature
4. (existing) mesh.member row exists for (meshId, pubkey) → else unauthorized

All rejection paths close the WS with code 1008 + structured error
message + metrics counter increment (connections_rejected_total by
reason).

new modules:
- apps/broker/src/crypto.ts: canonicalHello, verifyHelloSignature,
  HELLO_SKEW_MS constant
- apps/cli/src/crypto/hello-sig.ts: matching signHello helper

clients updated:
- apps/cli/src/ws/client.ts: signs hello before send
- apps/broker/scripts/{peer-a,peer-b}.ts (smoke-test): sign hellos
  with seed-provided secret keys

new regression tests — tests/hello-signature.test.ts (7):
- valid signature accepted
- bad signature (signed with wrong key) rejected
- timestamp too old rejected (>60s)
- timestamp too far in future rejected (>60s)
- tampered canonical field (different meshId at verify time) rejected
- malformed hex pubkey rejected
- malformed signature length rejected

verified live:
- apps/broker/scripts/smoke-test.sh: full hello+ack+send+push flow
- apps/cli/scripts/roundtrip.ts: signed hello + encrypted message
- 55/55 tests pass

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

apps/broker/src/crypto.ts

@@ -0,0 +1,79 @@
+/**
+ * Broker-side ed25519 verification helpers.
+ *
+ * Used to authenticate the WS hello handshake: clients sign a canonical
+ * byte string with their mesh.member.peerPubkey's secret key, broker
+ * verifies with the claimed pubkey, then cross-checks the pubkey is a
+ * current member of the claimed mesh.
+ */
+
+import sodium from "libsodium-wrappers";
+
+let ready = false;
+async function ensureSodium(): Promise<typeof sodium> {
+  if (!ready) {
+    await sodium.ready;
+    ready = true;
+  }
+  return sodium;
+}
+
+/** Canonical hello bytes: clients sign this, broker verifies this. */
+export function canonicalHello(
+  meshId: string,
+  memberId: string,
+  pubkey: string,
+  timestamp: number,
+): string {
+  return `${meshId}|${memberId}|${pubkey}|${timestamp}`;
+}
+
+export const HELLO_SKEW_MS = 60_000;
+
+/**
+ * Verify a hello's ed25519 signature + timestamp skew.
+ * Returns { ok: true } on success, or { ok: false, reason } describing
+ * which check failed (for structured error response).
+ */
+export async function verifyHelloSignature(args: {
+  meshId: string;
+  memberId: string;
+  pubkey: string;
+  timestamp: number;
+  signature: string;
+  now?: number;
+}): Promise<
+  | { ok: true }
+  | { ok: false; reason: "timestamp_skew" | "bad_signature" | "malformed" }
+> {
+  const now = args.now ?? Date.now();
+  if (
+    !Number.isFinite(args.timestamp) ||
+    Math.abs(now - args.timestamp) > HELLO_SKEW_MS
+  ) {
+    return { ok: false, reason: "timestamp_skew" };
+  }
+  if (
+    !/^[0-9a-f]{64}$/i.test(args.pubkey) ||
+    !/^[0-9a-f]{128}$/i.test(args.signature)
+  ) {
+    return { ok: false, reason: "malformed" };
+  }
+  const s = await ensureSodium();
+  try {
+    const canonical = canonicalHello(
+      args.meshId,
+      args.memberId,
+      args.pubkey,
+      args.timestamp,
+    );
+    const ok = s.crypto_sign_verify_detached(
+      s.from_hex(args.signature),
+      s.from_string(canonical),
+      s.from_hex(args.pubkey),
+    );
+    return ok ? { ok: true } : { ok: false, reason: "bad_signature" };
+  } catch {
+    return { ok: false, reason: "malformed" };
+  }
+}