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/e2e/replaySeal.ts

@@ -0,0 +1,49 @@
+/**
+ * Replay-frame sealer (recoverable K_content) — PLAN_RELAY_AGENT T19 (FIX 3).
+ *
+ * Live host→client frames use the EPHEMERAL DirectionalKeys.h2c (forward-secret, gone after
+ * reconnect). But "refresh the page and the Claude session is still there" needs the ring-buffer /
+ * preview ciphertext to be RECOVERABLE — so every replay-bound output is ALSO sealed under the
+ * host-scoped recoverable K_content = deriveContentKey({ hostContentSecret, sessionId, alg }),
+ * DISTINCT from the live h2c frame. The browser re-derives the identical K_content (P5) and opens
+ * it (P6). This is the single agent-side consumer of the FIX 3 recoverable key.
+ *
+ * INTEGRATION SEAM: the frozen §4.4 replay-crypto IMPLEMENTATIONS live in P4 `relay-e2e/`
+ * (`deriveContentKey`, `sealReplayFrame`). P4 is not built yet, so they are INJECTED here typed to
+ * the frozen relay-contracts signatures — production wiring passes the relay-e2e impls verbatim.
+ * `hostContentSecret` comes from Keystore.loadContentSecret() (T3); NEVER the ephemeral key,
+ * NEVER logged, NEVER sent to the relay (INV2/INV9).
+ */
+import type { AeadAlg, AeadKey, E2EEnvelope, ReplayKeyParams } from 'relay-contracts'
+
+/** The two §4.4 replay primitives, typed to the frozen relay-contracts signatures (P4 impls). */
+export interface ReplayCrypto {
+  deriveContentKey(params: ReplayKeyParams): AeadKey
+  sealReplayFrame(key: AeadKey, seq: bigint, plaintext: Uint8Array): E2EEnvelope
+}
+
+export interface ReplaySealer {
+  /** K_content seal with monotonic seq per session (INV13); NOT the live h2c frame. */
+  seal(plaintext: Uint8Array): E2EEnvelope
+}
+
+/**
+ * Build a per-(host, session) replay sealer. K_content is derived ONCE from
+ * { hostContentSecret, sessionId, alg }; seq is strictly monotonic from 0 (INV13).
+ */
+export function createReplaySealer(
+  hostContentSecret: Uint8Array,
+  sessionId: string,
+  alg: AeadAlg,
+  crypto: ReplayCrypto,
+): ReplaySealer {
+  const key = crypto.deriveContentKey({ hostContentSecret, sessionId, alg })
+  let seq = 0n
+  return {
+    seal(plaintext: Uint8Array): E2EEnvelope {
+      const env = crypto.sealReplayFrame(key, seq, plaintext)
+      seq += 1n
+      return env
+    },
+  }
+}