Simplify codebase: cleaner platform checks, modern fs API, remove boilerplate

[otiai10]

• Replace regex platform detection with direct process.platform === 'win32'
• Use fs.promises.access instead of manual callback-to-Promise wrapper
• Use bins.find(Boolean) instead of bins.find(bin => !!bin)
• Deduplicate PATHEXT extensions to avoid redundant access checks on non-Windows
• Remove 50+ lines of commented-out tsc --init boilerplate from tsconfig.json
• Reuse isWindows constant in tests instead of inline regex checks

https://claude.ai/code/session_01KtrZQqBfLaiYkKwvATqJCd

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (13e32c0) to head (54152a2).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #208   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines           22        24    +2     
  Branches        10         9    -1     
=========================================
+ Hits            22        24    +2     

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