fix: cross-platform CI test fixes and parse bug fix

- Add .gitattributes to force LF in mock files (Windows autocrlf)
- Fix parse.go: detect line endings from content instead of runtime.GOOS
- Add fail-fast: false to CI matrix; trigger on all branch pushes
- Skip chmod-based tests on Windows (permissions work differently)
- Use filepath.Join for expected paths in Windows path tests
- Use platform-appropriate invalid paths in error tests
- Add Windows absolute path test case for ValidateSheetName
- Skip Unix-specific integration tests on Windows

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/sheet/parse_extended_test.go

@@ -1,17 +1,11 @@
 package sheet
 
 import (
-	"runtime"
 	"testing"
 )
 
 // TestParseWindowsLineEndings tests parsing with Windows line endings
 func TestParseWindowsLineEndings(t *testing.T) {
-	// Only test Windows line endings on Windows
-	if runtime.GOOS != "windows" {
-		t.Skip("Skipping Windows line ending test on non-Windows platform")
-	}
-
 	// stub our cheatsheet content with Windows line endings
 	markdown := "---\r\nsyntax: go\r\ntags: [ test ]\r\n---\r\nTo foo the bar: baz"