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/test/x25519.test.ts

@@ -0,0 +1,40 @@
+import { describe, expect, it } from 'vitest'
+import { x25519 } from '@noble/curves/ed25519'
+import { deriveSharedSecret, generateEphemeralKeyPair } from '../src/x25519.js'
+import { E2EError } from '../src/errors.js'
+import { getWebCrypto } from '../src/crypto-provider.js'
+import { bytesToHex } from './helpers.js'
+
+describe('T5 x25519 ECDH', () => {
+  it('classic agreement: crossed pubkeys derive the same shared secret', async () => {
+    const a = await generateEphemeralKeyPair()
+    const b = await generateEphemeralKeyPair()
+    const ab = await deriveSharedSecret(a.privateKey, b.publicKey)
+    const ba = await deriveSharedSecret(b.privateKey, a.publicKey)
+    expect(ab.length).toBe(32)
+    expect(bytesToHex(ab)).toBe(bytesToHex(ba))
+  })
+
+  it('INV5: private key is a non-extractable CryptoKey (exportKey rejects)', async () => {
+    const a = await generateEphemeralKeyPair()
+    expect(a.privateKey.extractable).toBe(false)
+    await expect(getWebCrypto().exportKey('raw', a.privateKey)).rejects.toBeTruthy()
+  })
+
+  it('negative: an invalid peer public key is rejected with a typed error', async () => {
+    const a = await generateEphemeralKeyPair()
+    await expect(deriveSharedSecret(a.privateKey, new Uint8Array(8))).rejects.toBeInstanceOf(
+      E2EError,
+    )
+  })
+
+  it('parity: WebCrypto secret equals @noble/curves x25519 for the same key material', async () => {
+    // Generate a noble scalar, import it into WebCrypto as pkcs8, and check both agree.
+    const noblePriv = x25519.utils.randomPrivateKey()
+    const noblePub = x25519.getPublicKey(noblePriv)
+    const peer = await generateEphemeralKeyPair()
+    const nobleSecret = x25519.getSharedSecret(noblePriv, peer.publicKey).slice(-32)
+    const wcSecret = await deriveSharedSecret(peer.privateKey, noblePub)
+    expect(bytesToHex(wcSecret)).toBe(bytesToHex(nobleSecret))
+  })
+})