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/osfs/os_chroot.go

@@ -74,6 +74,10 @@ func (fs *ChrootOS) Remove(filename string) error {
 	return os.Remove(filename)
 }
 
+func (fs *ChrootOS) Chmod(path string, mode os.FileMode) error {
+	return os.Chmod(path, mode)
+}
+
 func (fs *ChrootOS) TempFile(dir, prefix string) (billy.File, error) {
 	if err := fs.createDir(dir + string(os.PathSeparator)); err != nil {
 		return nil, err