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/service/systemd.ts

@@ -0,0 +1,40 @@
+/**
+ * Linux systemd unit writer — PLAN_RELAY_AGENT T17. Runs as the LOGGED-IN USER (never root,
+ * EXPLORE §4d), restart-on-failure; no secrets in the unit (key/cert stay in the keystore).
+ */
+const UNIT_NAME = 'web-terminal-agent.service'
+
+export function systemdUnitName(): string {
+  return UNIT_NAME
+}
+
+export function systemdUnitPath(homedir: string): string {
+  return `${homedir}/.config/systemd/user/${UNIT_NAME}`
+}
+
+/** Build the unit. ExecStart = `<bin> run`; Restart=on-failure; User=<user> (never root). */
+export function buildSystemdUnit(binPath: string, user: string): string {
+  return [
+    '[Unit]',
+    'Description=web-terminal host agent (rendezvous relay)',
+    'After=network-online.target',
+    '',
+    '[Service]',
+    'Type=simple',
+    `ExecStart=${binPath} run`,
+    'Restart=on-failure',
+    'RestartSec=1',
+    `User=${user}`,
+    '',
+    '[Install]',
+    'WantedBy=default.target',
+    '',
+  ].join('\n')
+}
+
+export function systemdEnableCommand(): { cmd: string; args: readonly string[] } {
+  return { cmd: 'systemctl', args: ['--user', 'enable', '--now', UNIT_NAME] }
+}
+export function systemdDisableCommand(): { cmd: string; args: readonly string[] } {
+  return { cmd: 'systemctl', args: ['--user', 'disable', '--now', UNIT_NAME] }
+}