all: upgrade to Go 1.26rc2 and modernize codebase

This commit upgrades the codebase from Go 1.25.5 to Go 1.26rc2 and
adopts new language features.

Toolchain updates:
- go.mod: go 1.25.5 → go 1.26rc2
- flake.nix: buildGo125Module → buildGo126Module, go_1_25 → go_1_26
- flake.nix: build golangci-lint from source with Go 1.26
- Dockerfile.integration: golang:1.25-trixie → golang:1.26rc2-trixie
- Dockerfile.tailscale-HEAD: golang:1.25-alpine → golang:1.26rc2-alpine
- Dockerfile.derper: golang:alpine → golang:1.26rc2-alpine
- .goreleaser.yml: go mod tidy -compat=1.25 → -compat=1.26
- cmd/hi/run.go: fallback Go version 1.25 → 1.26rc2
- .pre-commit-config.yaml: simplify golangci-lint hook entry

Code modernization using Go 1.26 features:
- Replace tsaddr.SortPrefixes with slices.SortFunc + netip.Prefix.Compare
- Replace ptr.To(x) with new(x) syntax
- Replace errors.As with errors.AsType[T]

Lint rule updates:
- Add forbidigo rules to prevent regression to old patterns

hscontrol/handlers.go

@@ -36,8 +36,7 @@ const (
 
 // httpError logs an error and sends an HTTP error response with the given.
 func httpError(w http.ResponseWriter, err error) {
-	var herr HTTPError
-	if errors.As(err, &herr) {
+	if herr, ok := errors.AsType[HTTPError](err); ok {
 		http.Error(w, herr.Msg, herr.Code)
 		log.Error().Err(herr.Err).Int("code", herr.Code).Msgf("user msg: %s", herr.Msg)
 	} else {