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/helper/chroot/chroot.go

@@ -1,6 +1,7 @@
 package chroot
 
 import (
+	"errors"
 	"os"
 	"path/filepath"
 	"strings"
@@ -200,6 +201,19 @@ func (fs *ChrootHelper) Readlink(link string) (string, error) {
 	return string(os.PathSeparator) + target, nil
 }
 
+func (fs *ChrootHelper) Chmod(path string, mode os.FileMode) error {
+	fullpath, err := fs.underlyingPath(path)
+	if err != nil {
+		return err
+	}
+
+	c, ok := fs.underlying.(billy.Chmod)
+	if !ok {
+		return errors.New("underlying fs does not implement billy.Chmod")
+	}
+	return c.Chmod(fullpath, mode)
+}
+
 func (fs *ChrootHelper) Chroot(path string) (billy.Filesystem, error) {
 	fullpath, err := fs.underlyingPath(path)
 	if err != nil {