fix: replace setx with reg add to avoid 1024-char PATH truncation (#101)

- Use reg add instead of setx to bypass Windows 1024-character limit
- Add safety check for quotes/exclamation marks in PATH to prevent injection
- Preserve stderr output for better error diagnostics
- Update documentation with warnings about cmd PATH duplication
- Add test script for PATH update validation

Generated with SWE-Agent.ai

Co-Authored-By: SWE-Agent.ai <noreply@swe-agent.ai>

test_install_path.bat

@@ -0,0 +1,67 @@
+@echo off
+setlocal enabledelayedexpansion
+
+echo Testing PATH update with long strings...
+echo.
+
+rem Create a very long PATH string (over 1024 characters)
+set "LONG_PATH="
+for /L %%i in (1,1,30) do (
+    set "LONG_PATH=!LONG_PATH!C:\VeryLongDirectoryName%%i\SubDirectory\AnotherSubDirectory;"
+)
+
+echo Generated PATH length:
+echo !LONG_PATH! > temp_path.txt
+for %%A in (temp_path.txt) do set "PATH_LENGTH=%%~zA"
+del temp_path.txt
+echo !PATH_LENGTH! bytes
+
+rem Test 1: Verify reg add can handle long strings
+echo.
+echo Test 1: Testing reg add with long PATH...
+set "TEST_PATH=!LONG_PATH!%%USERPROFILE%%\bin"
+reg add "HKCU\Environment" /v TestPath /t REG_EXPAND_SZ /d "!TEST_PATH!" /f >nul 2>nul
+if errorlevel 1 (
+    echo FAIL: reg add failed with long PATH
+    goto :cleanup
+) else (
+    echo PASS: reg add succeeded with long PATH
+)
+
+rem Test 2: Verify the value was stored correctly
+echo.
+echo Test 2: Verifying stored value length...
+for /f "tokens=2*" %%A in ('reg query "HKCU\Environment" /v TestPath 2^>nul ^| findstr /I "TestPath"') do set "STORED_PATH=%%B"
+echo !STORED_PATH! > temp_stored.txt
+for %%A in (temp_stored.txt) do set "STORED_LENGTH=%%~zA"
+del temp_stored.txt
+echo Stored PATH length: !STORED_LENGTH! bytes
+
+if !STORED_LENGTH! LSS 1024 (
+    echo FAIL: Stored PATH was truncated
+    goto :cleanup
+) else (
+    echo PASS: Stored PATH was not truncated
+)
+
+rem Test 3: Verify %%USERPROFILE%%\bin is present
+echo.
+echo Test 3: Verifying %%USERPROFILE%%\bin is in stored PATH...
+echo !STORED_PATH! | findstr /I "USERPROFILE" >nul
+if errorlevel 1 (
+    echo FAIL: %%USERPROFILE%%\bin not found in stored PATH
+    goto :cleanup
+) else (
+    echo PASS: %%USERPROFILE%%\bin found in stored PATH
+)
+
+echo.
+echo ========================================
+echo All tests PASSED
+echo ========================================
+
+:cleanup
+echo.
+echo Cleaning up test registry key...
+reg delete "HKCU\Environment" /v TestPath /f >nul 2>nul
+endlocal