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_wasip1.go

@@ -0,0 +1,34 @@
+//go:build wasip1
+// +build wasip1
+
+package osfs
+
+import (
+	"os"
+	"syscall"
+)
+
+func (f *file) Lock() error {
+	f.m.Lock()
+	defer f.m.Unlock()
+	return nil
+}
+
+func (f *file) Unlock() error {
+	f.m.Lock()
+	defer f.m.Unlock()
+	return nil
+}
+
+func rename(from, to string) error {
+	return os.Rename(from, to)
+}
+
+// umask sets umask to a new value, and returns a func which allows the
+// caller to reset it back to what it was originally.
+func umask(new int) func() {
+	old := syscall.Umask(new)
+	return func() {
+		syscall.Umask(old)
+	}
+}