fix: prevent Windows ENOENT for codex availability checks

[Abdooo2235]

Summary

Fixes a Windows command resolution failure where Codex availability checks could fail with ENOENT even when codex was installed globally and available in PATH.

On Windows, command resolution can require shell execution for PATH-resolved shims. This change keeps Windows shell behavior and adds explicit regression coverage for it.

Closes #161.

Changes

• Updated command runner to make platform/spawn implementation injectable for tests while preserving behavior:
  • shell now resolves from options.shell ?? platform === "win32"
  • added platform and spawnSyncImpl test hooks
• Added regression tests to ensure:
  • Windows uses shell: true for PATH-resolved commands
  • non-Windows does not force shell mode

Validation

• npm test (pass)
• includes new tests in tests/process.test.mjs for Windows/non-Windows shell behavior

Risk

Low. Change is scoped to process invocation wiring and covered by targeted and full-suite tests.

[dkundel-openai]

Thanks this should have been fixed in #55