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/db/users_test.go

@@ -8,7 +8,6 @@ import (
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 	"gorm.io/gorm"
-	"tailscale.com/types/ptr"
 )
 
 func TestCreateAndDestroyUser(t *testing.T) {
@@ -79,7 +78,7 @@ func TestDestroyUserErrors(t *testing.T) {
 					Hostname:       "testnode",
 					UserID:         &user.ID,
 					RegisterMethod: util.RegisterMethodAuthKey,
-					AuthKeyID:      ptr.To(pak.ID),
+					AuthKeyID:      new(pak.ID),
 				}
 				trx := db.DB.Save(&node)
 				require.NoError(t, trx.Error)