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/golang.org/x/sys/unix/syscall_netbsd.go

@@ -248,6 +248,23 @@ func Statvfs(path string, buf *Statvfs_t) (err error) {
 	return Statvfs1(path, buf, ST_WAIT)
 }
 
+func Getvfsstat(buf []Statvfs_t, flags int) (n int, err error) {
+	var (
+		_p0     unsafe.Pointer
+		bufsize uintptr
+	)
+	if len(buf) > 0 {
+		_p0 = unsafe.Pointer(&buf[0])
+		bufsize = unsafe.Sizeof(Statvfs_t{}) * uintptr(len(buf))
+	}
+	r0, _, e1 := Syscall(SYS_GETVFSSTAT, uintptr(_p0), bufsize, uintptr(flags))
+	n = int(r0)
+	if e1 != 0 {
+		err = e1
+	}
+	return
+}
+
 /*
  * Exposed directly
  */