feat(relay): rendezvous-relay service — 7 packages + plans (contracts/transport/agent/control-plane/e2e/auth/web)

Multi-tenant reverse-tunnel service ("ngrok for Claude Code" with E2E): a
host-agent dials OUT to an operator-run relay; external devices reach the host
THROUGH the relay, routed by per-tenant subdomain, forwarding ciphertext only
(the relay never sees plaintext). Lets a customer reach their own self-hosted
web-terminal from anywhere with zero networking setup.

Packages — all tsc-strict + vitest green (656 tests), cross-package integration verified:
- relay-contracts: frozen shared contracts (mux frame codec, data model,
  capability token, E2E envelope, pairing) — the src/types.ts analog
- term-relay:   native WS mux + stateless data plane (subdomain routing, ciphertext forward)
- agent:        host-agent (pairing, per-host Ed25519 + mTLS dial-out, forwards to 127.0.0.1:3000)
- control-plane: accounts/hosts registry, pairing-code flow, routing table, provisioning
- relay-e2e:    browser<->agent E2E (X25519 ECDH through relay, AEAD, anti-replay, recoverable replay key)
- relay-auth:   Passkey/WebAuthn, capability tokens, per-host certs, deny-by-default tenant isolation
- relay-web:    browser login + Web Crypto E2E + client-side preview rendering

Security invariants INV1-15 enforced; cross-tenant isolation CI tripwire live
(.github/workflows/relay-tripwire.yml). Design + implementation-level plans in
docs/PLAN_RELAY_*.md and docs/EXPLORE_RELAY_SERVICE.md.

NOTE: generated autonomously per the reviewed plans. The security-critical
packages (relay-e2e, relay-auth) REQUIRE expert security audit before any real
deployment — passing tests prove self-consistency, not resistance to attackers.
Base app (src/, public/) unchanged; concurrent desktop work left uncommitted.

relay-e2e/src/x25519.ts

@@ -0,0 +1,47 @@
+/**
+ * T5 — ephemeral X25519 keygen + ECDH shared secret via WebCrypto.
+ *
+ * The private key is a NON-EXTRACTABLE `CryptoKey` (INV5): it never leaves as bytes —
+ * `exportKey('raw', priv)` rejects. Fresh per handshake (forward secrecy).
+ */
+import { getWebCrypto } from './crypto-provider.js'
+import { E2EError } from './errors.js'
+
+const ALG = 'X25519'
+const SHARED_SECRET_BITS = 256
+
+export interface EphemeralKeyPair {
+  readonly publicKey: Uint8Array
+  readonly privateKey: CryptoKey
+}
+
+/** Generate a fresh X25519 keypair; private key is non-extractable. */
+export async function generateEphemeralKeyPair(): Promise<EphemeralKeyPair> {
+  const subtle = getWebCrypto()
+  const pair = (await subtle.generateKey({ name: ALG }, false, ['deriveBits'])) as CryptoKeyPair
+  const rawPub = await subtle.exportKey('raw', pair.publicKey)
+  return { publicKey: new Uint8Array(rawPub), privateKey: pair.privateKey }
+}
+
+/** Derive the 32-byte raw ECDH shared secret from our private key + the peer's raw public key. */
+export async function deriveSharedSecret(
+  privateKey: CryptoKey,
+  peerPublicKey: Uint8Array,
+): Promise<Uint8Array> {
+  const subtle = getWebCrypto()
+  // Copy into a fresh ArrayBuffer-backed view so it satisfies DOM `BufferSource` (not SharedArrayBuffer).
+  const peerRaw = new Uint8Array(peerPublicKey.length)
+  peerRaw.set(peerPublicKey)
+  let peer: CryptoKey
+  try {
+    peer = await subtle.importKey('raw', peerRaw, { name: ALG }, false, [])
+  } catch {
+    throw new E2EError('E2E_BAD_PEER_KEY', 'peer X25519 public key is invalid')
+  }
+  try {
+    const bits = await subtle.deriveBits({ name: ALG, public: peer }, privateKey, SHARED_SECRET_BITS)
+    return new Uint8Array(bits)
+  } catch {
+    throw new E2EError('E2E_ECDH_FAILED', 'X25519 ECDH derivation failed')
+  }
+}