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-contracts/src/errors.ts

@@ -0,0 +1,37 @@
+/**
+ * Shared explicit error types for relay-contracts.
+ *
+ * `coding-style.md`: "Never silently swallow errors" — every codec throws a typed,
+ * descriptive error at the parse boundary instead of returning a partial/ambiguous value.
+ */
+
+/** Thrown when raw bytes / strings fail to decode into a frozen contract shape. */
+export class ContractDecodeError extends Error {
+  constructor(message: string) {
+    super(message)
+    this.name = 'ContractDecodeError'
+  }
+}
+
+/** Thrown when a value violates a frozen contract invariant before encoding. */
+export class ContractEncodeError extends Error {
+  constructor(message: string) {
+    super(message)
+    this.name = 'ContractEncodeError'
+  }
+}
+
+/**
+ * Thrown by crypto-shaped functions whose *implementation* lives in another relay
+ * package (P4 `relay-e2e/`, P5 `relay-auth/`). `relay-contracts` owns only the SHAPE
+ * (signature + types) per PLAN_RELAY_INDEX §2.1; the runtime is provided downstream.
+ */
+export class NotImplementedInContractsError extends Error {
+  constructor(fnName: string, owner: string) {
+    super(
+      `${fnName} is a frozen SIGNATURE only in relay-contracts; its crypto implementation lives in ${owner}. ` +
+        `Import the implementation from that package — do not call it from relay-contracts.`,
+    )
+    this.name = 'NotImplementedInContractsError'
+  }
+}