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/state/node_store_test.go

@@ -13,7 +13,6 @@ import (
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 	"tailscale.com/types/key"
-	"tailscale.com/types/ptr"
 )
 
 func TestSnapshotFromNodes(t *testing.T) {
@@ -175,7 +174,7 @@ func createTestNode(nodeID types.NodeID, userID uint, username, hostname string)
 		DiscoKey:   discoKey.Public(),
 		Hostname:   hostname,
 		GivenName:  hostname,
-		UserID:     ptr.To(userID),
+		UserID:     new(userID),
 		User: &types.User{
 			Name:        username,
 			DisplayName: username,
@@ -883,7 +882,7 @@ func createConcurrentTestNode(id types.NodeID, hostname string) types.Node {
 		Hostname:   hostname,
 		MachineKey: machineKey.Public(),
 		NodeKey:    nodeKey.Public(),
-		UserID:     ptr.To(uint(1)),
+		UserID:     new(uint(1)),
 		User: &types.User{
 			Name: "concurrent-test-user",
 		},