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/code-runtime/code-runtime-worker/tests/built-lib.e2e.ts

@@ -1,7 +1,7 @@
-import { spawn } from 'node:child_process'
 import { existsSync } from 'node:fs'
 import { join } from 'node:path'
 import { fileURLToPath } from 'node:url'
+import { execa } from 'execa'
 import { describe, expect, it } from 'vitest'
 
 /**
@@ -36,12 +36,13 @@ describe.skipIf(!built)('built lib real load path (plain node)', () => {
       console.log(JSON.stringify(result))
       process.exit(0)
     `
-    const child = spawn(process.execPath, ['--input-type=module', '-e', script], { cwd: pkgDir, stdio: ['ignore', 'pipe', 'pipe'] })
-    let stdout = ''
-    let stderr = ''
-    child.stdout.on('data', (chunk: Buffer) => { stdout += chunk.toString('utf8') })
-    child.stderr.on('data', (chunk: Buffer) => { stderr += chunk.toString('utf8') })
-    const exitCode = await new Promise<number | null>(resolve => child.on('close', resolve))
+    const { exitCode, stdout, stderr } = await execa(process.execPath, ['--input-type=module', '-e', script], {
+      cwd: pkgDir,
+      stdin: 'ignore',
+      timeout: 55_000,
+      killSignal: 'SIGKILL',
+      reject: false,
+    })
 
     expect(exitCode, `stderr:\n${stderr}`).toBe(0)
     const lastLine = stdout.trim().split('\n').at(-1) ?? ''