fix(bin): distinguish ENOENT from other spawn failures

[Arvuno]

Problem

bin/lazycodex-ai.js shells out to npx via spawnSync. When the user's PATH is missing npx (no Node.js installed, or a sandboxed environment that strips npx), the existing code prints the raw syscall error and exits 1. The error gives no hint that the actual problem is a missing npx, and no hint about what to do (install Node.js).

What changed

• Branch on result.error.code === "ENOENT" in bin/lazycodex-ai.js and print "npx not found on PATH. Install Node.js 20+ from https://nodejs.org." in that case.
• All other spawn errors fall through to the existing result.error.message print.
• Added test/bin-spawn-error.test.mjs to assert the new behavior. Test runs the bin with PATH="" to force ENOENT, captures stderr, and asserts the message mentions npx and Node.js. Test passes locally with node --test test/*.test.mjs.

Why this fits

Tiny, focused, no behavior change for valid input. Matches the style of the existing bin script (no external deps, plain Node).

Test

$ node --test test/*.test.mjs
ok 1 - bin/lazycodex-ai.js spawn error handling
# tests 1
# pass 1
# fail 0

Manual:

$ PATH= node bin/lazycodex-ai.js --help
npx not found on PATH. Install Node.js 20+ from https://nodejs.org.

No related issue — this is a clear UX gap observed by running the bin in a fresh container.

---

Summary by cubic

Improve bin error message when npx is not on PATH by detecting ENOENT and guiding users to install Node.js 20+. Adds a test to verify the behavior.

• Bug Fixes
  • Detect ENOENT from spawnSync and print "npx not found on PATH. Install Node.js 20+ from https://nodejs.org."; other errors stay the same.
  • Add test/bin-spawn-error.test.mjs to run with empty PATH, asserting the message and exit code 1.

^{Written for commit d40a0cb. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 2 files

_{Re-trigger cubic}

[code-yeongyu]

Verified the PR-specific test locally with node --test test/bin-spawn-error.test.mjs and confirmed GitGuardian + Cubic were PASS. Merged with a merge commit and deleted the branch. Thanks.

[Arvuno]

Thanks for the careful review and the merge, @code-yeongyu 🙌

If you run into a similar spawnSync ENOENT vs "command not found" ambiguity elsewhere in the CLI, this pattern (out.includes('ENOENT') → CodexNotFoundError, else CodexExecError) generalizes cleanly — happy to follow up with a second PR if it shows up in another code path.