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}.

packages/examples/acp-demo/tests/built-bin.e2e.ts

@@ -17,6 +17,7 @@ import {
 import { Readable, Writable } from 'node:stream'
 import { promisify } from 'node:util'
 import { zstdDecompress } from 'node:zlib'
+import { execa } from 'execa'
 import { afterEach, describe, expect, it } from 'vitest'
 
 /**
@@ -209,25 +210,19 @@ describe.skipIf(!existsSync(acpBin))('dsh-acp-demo BUILT bin (node lib/bin.js, n
   }, 30_000)
 })
 
-/** Spawn the built acp bin against `configArg` and resolve with its exit code + stderr. */
-function runBinExpectingExit(configArg: string, cwd: string = tmpdir()): Promise<{ code: number; stderr: string }> {
-  return new Promise((resolve, reject) => {
-    const proc = spawn(process.execPath, [acpBin, '--config', configArg], {
-      cwd,
-      env: {
-        ...process.env,
-        DSH_HOME: join(cwd, '.dsh'),
-        DSH_AGENTS_HOME: join(cwd, '.agents'),
-      },
-      stdio: ['pipe', 'pipe', 'pipe'],
-    })
-    child = proc
-    let stderr = ''
-    proc.stderr.setEncoding('utf8')
-    proc.stderr.on('data', (c: string) => { stderr += c })
-    const timer = setTimeout(() => { proc.kill('SIGKILL'); reject(new Error(`bin did not exit within 25s. stderr:\n${stderr}`)) }, 25_000)
-    proc.on('exit', (code) => { clearTimeout(timer); resolve({ code: code ?? -1, stderr }) })
-    proc.on('error', (err) => { clearTimeout(timer); reject(err) })
-    proc.stdin.end()
+/** Spawn the built acp bin against `configArg` (stdin closed at EOF) and resolve with its exit code + stderr. */
+async function runBinExpectingExit(configArg: string, cwd: string = tmpdir()): Promise<{ code: number; stderr: string }> {
+  const result = await execa(process.execPath, [acpBin, '--config', configArg], {
+    cwd,
+    env: {
+      DSH_HOME: join(cwd, '.dsh'),
+      DSH_AGENTS_HOME: join(cwd, '.agents'),
+    },
+    input: '',
+    timeout: 25_000,
+    killSignal: 'SIGKILL',
+    reject: false,
   })
+  if (result.timedOut) throw new Error(`bin did not exit within 25s. stderr:\n${result.stderr}`)
+  return { code: result.exitCode ?? -1, stderr: result.stderr }
 }