test: adopt execa for hand-rolled subprocess plumbing, parseArgs for llm-mock-server CLI, vi.waitFor for poll loops

Implements the execa Agent Note's four sub-changes:
- execa (root devDep + loader-smoke dep) replaces the hand-rolled
  spawn-collect-timeout choreography in loader-smoke, apps/cli and
  cli-demo/acp-demo built-bin e2e, lsp-local and code-runtime-worker
  built-lib e2e, the tui pty-harness outer collector, the jsonrpc
  keyless smoke, and crash-recovery's child spawn. Genuinely custom
  parts stay custom: cli-demo's interrupt-on-marker, jsonrpc's
  line-predicate protocol driving, crash-recovery's SIGKILL-at-failpoint.
  The two loader-smoke /* v8 ignore */ OS-error branches are gone.
- llm-mock-server CLI tokenizes via node:util parseArgs; numeric
  coercion/bounds/cross-option constraints stay manual; pinned
  error-message tests updated to the parseArgs texts.
- both loadRootEnv copies in apps/web/tests are deleted: the owning
  vitest configs (web unconditionally, snapshot in record mode)
  already load the repo-root .env before these files run.
- the four poll loops (acp-snapshot harness waits + crash-recovery
  waitForFile) ride vi.waitFor with explicit {interval, timeout}.

apps/web/tests/scaffold.ts

@@ -17,7 +17,7 @@
 // the open llm seam post-boot with installLlmReplay on the settled root ctx
 // (the plugin-row path discards the ReplayHandle; the direct install keeps
 // assertConsumed for the teardown fixture-consumption check).
-import { existsSync, readFileSync } from 'node:fs'
+import { existsSync } from 'node:fs'
 import { mkdtemp, readFile, readdir, rm, utimes, writeFile } from 'node:fs/promises'
 import { tmpdir } from 'node:os'
 import { join, resolve } from 'node:path'
@@ -63,16 +63,6 @@ const CONFIG_PATH = join(REPO_ROOT, 'apps/cli/cordis.yml')
 // contextWindow keeps that pressure path provably inert for small fixtures.
 const REPLAY_PROVIDERS = [{ id: 'deepseek', name: 'DeepSeek', models: [{ id: 'deepseek-v4-flash', contextWindow: 128_000 }] }]
 
-/** Repo-root .env → process.env for record mode (never overrides set vars); the smoke-real convention. */
-function loadRootEnv(): void {
-  const envPath = join(REPO_ROOT, '.env')
-  if (!existsSync(envPath)) return
-  for (const line of readFileSync(envPath, 'utf8').split('\n')) {
-    const m = /^([A-Za-z_][A-Za-z0-9_]*)=(.*)$/.exec(line.trim())
-    if (m !== null && process.env[m[1]!] === undefined) process.env[m[1]!] = m[2]
-  }
-}
-
 /** A booted web scaffold: real composition, mode-selected model backend, temp world. */
 export interface WebScaffold {
   /** The active snapshot mode this scaffold booted under. */
@@ -123,7 +113,8 @@ export async function launchWebScaffold(options: LaunchOptions = {}): Promise<We
   requireDist()
   const mode = webSnapshotMode()
   if (mode === 'record') {
-    loadRootEnv()
+    // Both owning vitest configs (web unconditionally, snapshot in record
+    // mode) load the repo-root .env before this file runs.
     if (process.env.DEEPSEEK_API_KEY === undefined || process.env.DEEPSEEK_API_KEY.length === 0) {
       throw new Error('web e2e record mode needs DEEPSEEK_API_KEY (env or repo-root .env)')
     }