feat(agent): inject S0 env into launchd/systemd via install CLI (S2)

- launchd writer emits <EnvironmentVariables>; systemd writer emits EnvironmentFile/Environment;
  originConfig zone parameterized (default 'term' preserved for relay callers).
- InstallOptions threaded CLI install -> createCliDeps -> installService seam -> writers, so generated
  units carry BIND_HOST (defaults 127.0.0.1 for tunnel hosts), ALLOWED_ORIGINS, PORT, SHELL_PATH,
  IDLE_TTL, USE_TMUX. systemd rejects control chars in env values.
Verified: agent typecheck+build clean; vitest 166/166 (154 baseline + 12 new).

agent/src/cli.ts

@@ -6,6 +6,7 @@
 import type { AgentConfig } from './config/agentConfig.js'
 import type { AgentIdentity } from './keys/identity.js'
 import type { Keystore } from './keys/keystore.js'
+import type { InstallOptions } from './service/install.js'
 import type { EnrollResult } from 'relay-contracts'
 
 export type CliCommand = 'pair' | 'run' | 'status' | 'install' | 'uninstall'
@@ -30,7 +31,9 @@ export interface CliDeps {
   generateIdentity(): AgentIdentity
   redeem(cfg: AgentConfig, code: string, id: AgentIdentity, ks: Keystore): Promise<EnrollResult>
   runTunnel(cfg: AgentConfig, ks: Keystore): Promise<number>
-  installService(cfg: AgentConfig): Promise<void>
+  /** Resolve per-host install inputs (S0 env incl. loopback BIND_HOST, tunnel origin) from env/flags. */
+  resolveInstallOptions(): InstallOptions
+  installService(cfg: AgentConfig, options: InstallOptions): Promise<void>
   uninstallService(): Promise<void>
   print(line: string): void
 }
@@ -70,7 +73,7 @@ export async function runCli(args: CliArgs, deps: CliDeps): Promise<number> {
       ks.saveIdentity(id)
       const enroll = await deps.redeem(cfg, args.code!, id, ks)
       deps.print(`paired: host ${enroll.hostId} subdomain ${enroll.subdomain}`)
-      if (args.flags['install']) await deps.installService(cfg)
+      if (args.flags['install']) await deps.installService(cfg, deps.resolveInstallOptions())
       return 0
     }
     case 'run': {
@@ -88,7 +91,7 @@ export async function runCli(args: CliArgs, deps: CliDeps): Promise<number> {
       return 0
     }
     case 'install':
-      await deps.installService(cfg)
+      await deps.installService(cfg, deps.resolveInstallOptions())
       return 0
     case 'uninstall':
       await deps.uninstallService()