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.

agent/src/keys/keystore.ts

@@ -0,0 +1,109 @@
+/**
+ * On-disk keystore — PLAN_RELAY_AGENT T3 (INV4/INV5). Persists ONLY to `stateDir`, every secret
+ * file mode `0600`. Stores: the Ed25519 private key (PKCS#8 PEM), the mTLS cert + CA chain, and
+ * the FIX 3 unwrapped `hostContentSecret`. The raw pairing code is NEVER written here (T4).
+ */
+import {
+  chmodSync,
+  existsSync,
+  mkdirSync,
+  readFileSync,
+  statSync,
+  writeFileSync,
+} from 'node:fs'
+import { join } from 'node:path'
+import type { AgentIdentity } from './identity.js'
+import { identityFromPrivatePem } from './identity.js'
+
+const SECRET_MODE = 0o600
+const DIR_MODE = 0o700
+
+export interface Keystore {
+  saveIdentity(id: AgentIdentity): void
+  loadIdentity(): AgentIdentity | null
+  saveCert(certPem: string, caChainPem: string): void
+  loadCert(): { certPem: string; caChainPem: string } | null
+  saveContentSecret(secret: Uint8Array): void
+  loadContentSecret(): Uint8Array | null
+}
+
+const KEY_FILE = 'agent.key.pem'
+const CERT_FILE = 'agent.cert.pem'
+const CA_FILE = 'agent.ca.pem'
+const CONTENT_SECRET_FILE = 'content.secret'
+
+/** Corrupt/unreadable key material surfaces as a typed error (never a silent empty read). */
+export class KeystoreError extends Error {
+  constructor(message: string) {
+    super(message)
+    this.name = 'KeystoreError'
+  }
+}
+
+function ensureDir(stateDir: string): void {
+  if (!existsSync(stateDir)) {
+    mkdirSync(stateDir, { recursive: true, mode: DIR_MODE })
+    return
+  }
+  // Refuse to write secrets into a world-/group-accessible directory (INV5 hard error).
+  const mode = statSync(stateDir).mode & 0o777
+  if ((mode & 0o077) !== 0) {
+    throw new KeystoreError(
+      `stateDir ${stateDir} is group/world accessible (mode ${mode.toString(8)}); refusing to write secrets`,
+    )
+  }
+}
+
+function writeSecret(path: string, data: string | Uint8Array): void {
+  writeFileSync(path, data, { mode: SECRET_MODE })
+  // Enforce 0600 even if a prior umask/file left it wider.
+  chmodSync(path, SECRET_MODE)
+}
+
+export function openKeystore(stateDir: string): Keystore {
+  const keyPath = join(stateDir, KEY_FILE)
+  const certPath = join(stateDir, CERT_FILE)
+  const caPath = join(stateDir, CA_FILE)
+  const secretPath = join(stateDir, CONTENT_SECRET_FILE)
+
+  return {
+    saveIdentity(id: AgentIdentity): void {
+      ensureDir(stateDir)
+      writeSecret(keyPath, id.exportPrivatePkcs8Pem())
+    },
+    loadIdentity(): AgentIdentity | null {
+      if (!existsSync(keyPath)) return null
+      let pem: string
+      try {
+        pem = readFileSync(keyPath, 'utf8')
+      } catch (err) {
+        throw new KeystoreError(`failed to read identity key: ${(err as Error).message}`)
+      }
+      try {
+        return identityFromPrivatePem(pem)
+      } catch (err) {
+        throw new KeystoreError(`corrupt identity key file: ${(err as Error).message}`)
+      }
+    },
+    saveCert(certPem: string, caChainPem: string): void {
+      ensureDir(stateDir)
+      writeSecret(certPath, certPem)
+      writeSecret(caPath, caChainPem)
+    },
+    loadCert(): { certPem: string; caChainPem: string } | null {
+      if (!existsSync(certPath) || !existsSync(caPath)) return null
+      return {
+        certPem: readFileSync(certPath, 'utf8'),
+        caChainPem: readFileSync(caPath, 'utf8'),
+      }
+    },
+    saveContentSecret(secret: Uint8Array): void {
+      ensureDir(stateDir)
+      writeSecret(secretPath, secret)
+    },
+    loadContentSecret(): Uint8Array | null {
+      if (!existsSync(secretPath)) return null
+      return new Uint8Array(readFileSync(secretPath))
+    },
+  }
+}