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/policy_test.go

@@ -14,7 +14,6 @@ import (
 	"github.com/stretchr/testify/require"
 	"gorm.io/gorm"
 	"tailscale.com/tailcfg"
-	"tailscale.com/types/ptr"
 )
 
 var ap = func(ipStr string) *netip.Addr {
@@ -1084,21 +1083,21 @@ func TestSSHPolicyRules(t *testing.T) {
 	nodeUser1 := types.Node{
 		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]),
 	}
 	nodeUser2 := types.Node{
 		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]),
 	}
 
 	taggedClient := types.Node{
 		Hostname: "tagged-client",
 		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:client"},
 	}
 
@@ -1106,8 +1105,8 @@ func TestSSHPolicyRules(t *testing.T) {
 	nodeTaggedServer := types.Node{
 		Hostname: "tagged-server",
 		IPv4:     ap("100.64.0.5"),
-		UserID:   ptr.To(uint(1)),
-		User:     ptr.To(users[0]),
+		UserID:   new(uint(1)),
+		User:     new(users[0]),
 		Tags:     []string{"tag:server"},
 	}