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_hurd.go

@@ -11,6 +11,7 @@ package unix
 int ioctl(int, unsigned long int, uintptr_t);
 */
 import "C"
+import "unsafe"
 
 func ioctl(fd int, req uint, arg uintptr) (err error) {
 	r0, er := C.ioctl(C.int(fd), C.ulong(req), C.uintptr_t(arg))