chore: modernize CI and update Go toolchain

- Bump Go from 1.19 to 1.26 and update all dependencies
- Rewrite CI workflow with matrix strategy (Linux, macOS, Windows)
- Update GitHub Actions to current versions (checkout@v4, setup-go@v5)
- Update CodeQL actions from v1 to v3
- Fix cross-platform bug in mock/path.go (path.Join -> filepath.Join)
- Clean up dependabot config (weekly schedule, remove stale ignore)

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

vendor/github.com/go-git/go-billy/v5/memfs/storage.go

@@ -6,6 +6,7 @@ import (
 	"io"
 	"os"
 	"path/filepath"
+	"strings"
 	"sync"
 )
 
@@ -112,7 +113,7 @@ func (s *storage) Rename(from, to string) error {
 	move := [][2]string{{from, to}}
 
 	for pathFrom := range s.files {
-		if pathFrom == from || !filepath.HasPrefix(pathFrom, from) {
+		if pathFrom == from || !strings.HasPrefix(pathFrom, from) {
 			continue
 		}
 
@@ -168,6 +169,18 @@ func (s *storage) Remove(path string) error {
 	return nil
 }
 
+func (s *storage) Chmod(path string, mode os.FileMode) error {
+	path = clean(path)
+
+	f, has := s.Get(path)
+	if !has {
+		return os.ErrNotExist
+	}
+
+	f.mode = mode
+	return nil
+}
+
 func clean(path string) string {
 	return filepath.Clean(filepath.FromSlash(path))
 }