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/support/llm-mock-server/tests/cli.spec.ts

@@ -101,8 +101,11 @@ describe('mock LLM server CLI parser', () => {
 
   it.each([
     [[], /--sequence is required/],
-    [['--wat'], /requires a value/],
-    [['--wat', 'x'], /unknown option/],
+    // Tokenizer-level failures carry node:util parseArgs's own messages.
+    [['--wat'], /Unknown option '--wat'/],
+    [['--wat', 'x'], /Unknown option '--wat'/],
+    [['--port'], /Option '--port <value>' argument missing/],
+    [['--sequence', 'success', 'stray'], /Unexpected argument 'stray'/],
     [['--port', 'NaN', '--sequence', 'success'], /finite number/],
     [['--sequence', 'success,'], /non-empty/],
     [['--sequence', 'success,connection_refused'], /only as the first/],
@@ -110,7 +113,8 @@ describe('mock LLM server CLI parser', () => {
     [['--sequence', 'unknown'], /unknown behavior/],
     [['--sequence', 'connection_refused,success', '--port', '0'], /nonzero/],
     [['--sequence', 'success', '--listen-delay-ms', '5'], /requires connection_refused/],
-    [['--sequence', 'connection_refused,success', '--listen-delay-ms', '-1'], /integer between 0 and 2147483647/],
+    // `=` syntax: a space-separated leading-dash value is a tokenizer error, not a bounds probe.
+    [['--sequence', 'connection_refused,success', '--listen-delay-ms=-1'], /integer between 0 and 2147483647/],
     [['--sequence', 'connection_refused,success', '--listen-delay-ms', '1.5'], /integer between 0 and 2147483647/],
     [['--sequence', 'connection_refused,success', '--listen-delay-ms', '2147483648'], /integer between 0 and 2147483647/],
     [['--sequence', 'success', '--seed', '1'], /require random/],