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/fs.go

@@ -128,12 +128,18 @@ type Symlink interface {
 	Readlink(link string) (string, error)
 }
 
-// Change abstract the FileInfo change related operations in a storage-agnostic
-// interface as an extension to the Basic interface
-type Change interface {
+// Chmod abstracts the logic around changing file modes.
+type Chmod interface {
 	// Chmod changes the mode of the named file to mode. If the file is a
 	// symbolic link, it changes the mode of the link's target.
 	Chmod(name string, mode os.FileMode) error
+}
+
+// Change abstract the FileInfo change related operations in a storage-agnostic
+// interface as an extension to the Basic interface
+type Change interface {
+	Chmod
+
 	// Lchown changes the numeric uid and gid of the named file. If the file is
 	// a symbolic link, it changes the uid and gid of the link itself.
 	Lchown(name string, uid, gid int) error
@@ -164,6 +170,8 @@ type File interface {
 	// Name returns the name of the file as presented to Open.
 	Name() string
 	io.Writer
+	// TODO: Add io.WriterAt for v6  
+	// io.WriterAt
 	io.Reader
 	io.ReaderAt
 	io.Seeker