chore: replace which with spawnSync

[trivikr]

Use Node.js' built-in child_process API to detect whether the GitHub CLI is available.
This removes the runtime dependency for "which".

Testing

when gh is installed

$> node
Welcome to Node.js v22.23.2.
Type ".help" for more information.
> require("which").sync('gh', { nothrow: true }) !== null
true
> require('node:child_process').spawnSync(
...   'gh', ['--version'], { stdio: 'ignore' }
... ).status === 0
true

when gh is not installed

$> node
Welcome to Node.js v22.23.2.
Type ".help" for more information.
> require("which").sync('gh', { nothrow: true }) !== null
false
> require('node:child_process').spawnSync(
...   'gh', ['--version'], { stdio: 'ignore' }
... ).status === 0
false

[codecov]

Codecov Report

❌ Patch coverage is 25.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 57.81%. Comparing base (c53f184) to head (24bf247).

Files with missing lines	Patch %	Lines
lib/utils.js	25.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1165      +/-   ##
==========================================
- Coverage   57.82%   57.81%   -0.02%     
==========================================
  Files          53       53              
  Lines       10395    10397       +2     
==========================================
  Hits         6011     6011              
- Misses       4384     4386       +2     

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