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/memory.go

@@ -9,6 +9,7 @@ import (
 	"path/filepath"
 	"sort"
 	"strings"
+	"syscall"
 	"time"
 
 	"github.com/go-git/go-billy/v5"
@@ -18,16 +19,19 @@ import (
 
 const separator = filepath.Separator
 
-// Memory a very convenient filesystem based on memory files
+var errNotLink = errors.New("not a link")
+
+// Memory a very convenient filesystem based on memory files.
 type Memory struct {
 	s *storage
 
 	tempCount int
 }
 
-//New returns a new Memory filesystem.
+// New returns a new Memory filesystem.
 func New() billy.Filesystem {
 	fs := &Memory{s: newStorage()}
+	fs.s.New("/", 0755|os.ModeDir, 0)
 	return chroot.New(fs, string(separator))
 }
 
@@ -57,7 +61,9 @@ func (fs *Memory) OpenFile(filename string, flag int, perm os.FileMode) (billy.F
 		}
 
 		if target, isLink := fs.resolveLink(filename, f); isLink {
-			return fs.OpenFile(target, flag, perm)
+			if target != filename {
+				return fs.OpenFile(target, flag, perm)
+			}
 		}
 	}
 
@@ -68,8 +74,6 @@ func (fs *Memory) OpenFile(filename string, flag int, perm os.FileMode) (billy.F
 	return f.Duplicate(filename, perm, flag), nil
 }
 
-var errNotLink = errors.New("not a link")
-
 func (fs *Memory) resolveLink(fullpath string, f *file) (target string, isLink bool) {
 	if !isSymlink(f.mode) {
 		return fullpath, false
@@ -131,8 +135,12 @@ func (a ByName) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }
 func (fs *Memory) ReadDir(path string) ([]os.FileInfo, error) {
 	if f, has := fs.s.Get(path); has {
 		if target, isLink := fs.resolveLink(path, f); isLink {
-			return fs.ReadDir(target)
+			if target != path {
+				return fs.ReadDir(target)
+			}
 		}
+	} else {
+		return nil, &os.PathError{Op: "open", Path: path, Err: syscall.ENOENT}
 	}
 
 	var entries []os.FileInfo
@@ -169,17 +177,23 @@ func (fs *Memory) Remove(filename string) error {
 	return fs.s.Remove(filename)
 }
 
+func (fs *Memory) Chmod(path string, mode os.FileMode) error {
+	return fs.s.Chmod(path, mode)
+}
+
+// Falls back to Go's filepath.Join, which works differently depending on the
+// OS where the code is being executed.
 func (fs *Memory) Join(elem ...string) string {
 	return filepath.Join(elem...)
 }
 
 func (fs *Memory) Symlink(target, link string) error {
-	_, err := fs.Stat(link)
+	_, err := fs.Lstat(link)
 	if err == nil {
 		return os.ErrExist
 	}
 
-	if !os.IsNotExist(err) {
+	if !errors.Is(err, os.ErrNotExist) {
 		return err
 	}
 
@@ -230,7 +244,7 @@ func (f *file) Read(b []byte) (int, error) {
 	n, err := f.ReadAt(b, f.position)
 	f.position += int64(n)
 
-	if err == io.EOF && n != 0 {
+	if errors.Is(err, io.EOF) && n != 0 {
 		err = nil
 	}
 
@@ -269,6 +283,10 @@ func (f *file) Seek(offset int64, whence int) (int64, error) {
 }
 
 func (f *file) Write(p []byte) (int, error) {
+	return f.WriteAt(p, f.position)
+}
+
+func (f *file) WriteAt(p []byte, off int64) (int, error) {
 	if f.isClosed {
 		return 0, os.ErrClosed
 	}
@@ -277,8 +295,8 @@ func (f *file) Write(p []byte) (int, error) {
 		return 0, errors.New("write not supported")
 	}
 
-	n, err := f.content.WriteAt(p, f.position)
-	f.position += int64(n)
+	n, err := f.content.WriteAt(p, off)
+	f.position = off + int64(n)
 
 	return n, err
 }

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))
 }