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/policy/route_approval_test.go

@@ -10,7 +10,6 @@ import (
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 	"gorm.io/gorm"
-	"tailscale.com/types/ptr"
 )
 
 func TestNodeCanApproveRoute(t *testing.T) {
@@ -25,24 +24,24 @@ func TestNodeCanApproveRoute(t *testing.T) {
 		ID:       1,
 		Hostname: "user1-device",
 		IPv4:     ap("100.64.0.1"),
-		UserID:   ptr.To(uint(1)),
-		User:     ptr.To(users[0]),
+		UserID:   new(uint(1)),
+		User:     new(users[0]),
 	}
 
 	exitNode := types.Node{
 		ID:       2,
 		Hostname: "user2-device",
 		IPv4:     ap("100.64.0.2"),
-		UserID:   ptr.To(uint(2)),
-		User:     ptr.To(users[1]),
+		UserID:   new(uint(2)),
+		User:     new(users[1]),
 	}
 
 	taggedNode := types.Node{
 		ID:       3,
 		Hostname: "tagged-server",
 		IPv4:     ap("100.64.0.3"),
-		UserID:   ptr.To(uint(3)),
-		User:     ptr.To(users[2]),
+		UserID:   new(uint(3)),
+		User:     new(users[2]),
 		Tags:     []string{"tag:router"},
 	}
 
@@ -50,8 +49,8 @@ func TestNodeCanApproveRoute(t *testing.T) {
 		ID:       4,
 		Hostname: "multi-tag-node",
 		IPv4:     ap("100.64.0.4"),
-		UserID:   ptr.To(uint(2)),
-		User:     ptr.To(users[1]),
+		UserID:   new(uint(2)),
+		User:     new(users[1]),
 		Tags:     []string{"tag:router", "tag:server"},
 	}