fix: Windows compatibility - use shutil.which for ripgrep, add UTF-8 encoding

[RudyCity]

Problem

FastContext tools fail on Windows with two errors:

Issue	Error	Root Cause
Grep fails	[WinError 2] The system cannot find the file specified	_rg_path = "/usr/bin/rg" is hardcoded to a Linux path
Read fails	'charmap' codec can't decode byte 0x81	aiofiles.open() has no encoding specified; Windows defaults to cp1252

Fix

grep.py

• Replace hardcoded _rg_path = "/usr/bin/rg" with shutil.which("rg") or "rg" (cross-platform)
• Add encoding="utf-8", errors="replace" to subprocess.run()

glob.py

• Replace bare "rg" with shutil.which("rg") or "rg" (explicit resolution)
• Add encoding="utf-8", errors="replace" to subprocess.run()

read.py

• Add encoding="utf-8", errors="replace" to aiofiles.open()

Testing

• Verified shutil.which("rg") resolves correctly on Windows
• Confirmed old /usr/bin/rg path does not exist on Windows (root cause of WinError 2)
• Verified charmap crash occurs without encoding fix on non-ASCII content
• Verified encoding="utf-8", errors="replace" reads all content without crash
• All three tool modules import successfully after changes

[RudyCity]

Pushed additional commits with more Windows compatibility fixes:

• Replace Path.as_posix() with str() for native Windows paths in subprocess cwd
• Add friendly error message when ripgrep (rg) is not installed instead of cryptic FileNotFoundError
• Detect Windows shell via COMSPEC env var instead of defaulting to bash in system prompt
• Add encoding='utf-8' to all open(), read_text(), write_text(), and subprocess.run() calls in benchmark scripts

[RudyCity]

@microsoft-github-policy-service agree