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/ioctl_signed.go

@@ -6,9 +6,7 @@
 
 package unix
 
-import (
-	"unsafe"
-)
+import "unsafe"
 
 // ioctl itself should not be exposed directly, but additional get/set
 // functions for specific types are permissible.
@@ -28,6 +26,13 @@ func IoctlSetPointerInt(fd int, req int, value int) error {
 	return ioctlPtr(fd, req, unsafe.Pointer(&v))
 }
 
+// IoctlSetString performs an ioctl operation which sets a string value
+// on fd, using the specified request number.
+func IoctlSetString(fd int, req int, value string) error {
+	bs := append([]byte(value), 0)
+	return ioctlPtr(fd, req, unsafe.Pointer(&bs[0]))
+}
+
 // IoctlSetWinsize performs an ioctl on fd with a *Winsize argument.
 //
 // To change fd's window size, the req argument should be TIOCSWINSZ.