feat(inquirerer-test): add runCli subprocess helper + Testing docs

[pyramation]

Summary

Adds a runCli subprocess helper to @inquirerer/test and surfaces the package from inquirerer's own README so it's actually discoverable.

What's new in @inquirerer/test (sibling to the existing in-process createTestEnvironment/createTestFixture):

• runCli(bin, args, options?) — spawn a CLI binary, capture stdout/stderr, enforce a timeout, and assert on exit code. Defaults match common Jest expectations: rejects on non-zero exit with captured streams attached to RunCliError; pass reject: false to inspect failures without try/catch.
• parseArgString(args) — split a shell-like string into an argv array (whitespace + quote-aware, no shell expansion).
• RunCliError — typed error with stdout / stderr / exitCode / signal / timedOut / command fields.

Why now

Hand-rolled subprocess harnesses keep showing up (spawn + arg-string parser + timeout + stdout/stderr capture) any time someone wants a real CLI E2E test. The in-process createTestEnvironment is great for unit-testing prompts but doesn't help when the test needs the actual built executable. Now both shapes live in one package.

Inquirerer README discoverability

The companion test package was effectively invisible from inquirerer's docs — no Testing section, no link. Added a small Testing section that shows both the in-process and subprocess flows and links to the full @inquirerer/test README.

Files

• packages/inquirerer-test/src/subprocess.ts — new helper
• packages/inquirerer-test/src/index.ts — re-exports runCli, parseArgString, RunCliError, RunCliOptions, RunCliResult
• packages/inquirerer-test/__tests__/subprocess.test.ts — 14 tests covering parseArgString (5) and runCli (9: stdout/stderr capture, default reject behaviour, error attachment, reject: false, stdin piping, custom cwd/env, timeout, spawn error)
• packages/inquirerer-test/README.md — new "Subprocess Testing (CLI E2E)" section + API table
• packages/inquirerer/README.md — new "Testing" section pointing at @inquirerer/test

Review & Testing Checklist for Human

• Skim subprocess.ts API surface — happy with runCli(bin, args, options) signature, default 30s timeout, default reject: true, RunCliError shape?
• Confirm the docstring example for env (replaces parent env when set) matches the policy you want — the alternative would be "merge with parent" but I matched Node's child_process.spawn semantics for least surprise.
• Confirm package version bump policy — this is a feature add so should land as a minor (1.4.0); leaving the version at 1.3.5 for whoever's running lerna version.

Notes

• 14/14 tests pass locally (pnpm --filter @inquirerer/test test).
• Build passes (pnpm --filter @inquirerer/test build) — both CJS and ESM.
• Follow-up downstream PRs (separate, not in this PR):
  • constructive-io/constructive: drop the local test-utils re-export and import from @inquirerer/test directly.
  • constructive-io/agentic-db: replace the hand-rolled runCli in cli-e2e-tests with this one.

Link to Devin session: https://app.devin.ai/sessions/81001448b7c54b8099437f706d44a30d
Requested by: @pyramation

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