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.go

@@ -2,7 +2,6 @@ package sheet
 
 import (
 	"fmt"
-	"runtime"
 	"strings"
 
 	"gopkg.in/yaml.v3"
@@ -11,9 +10,9 @@ import (
 // Parse parses cheatsheet frontmatter
 func parse(markdown string) (frontmatter, string, error) {
 
-	// determine the appropriate line-break for the platform
+	// detect the line-break style used in the content
 	linebreak := "\n"
-	if runtime.GOOS == "windows" {
+	if strings.Contains(markdown, "\r\n") {
 		linebreak = "\r\n"
 	}