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.go

@@ -9,7 +9,6 @@ import (
 	"github.com/juanfont/headscale/hscontrol/util"
 	"github.com/rs/zerolog/log"
 	"github.com/samber/lo"
-	"tailscale.com/net/tsaddr"
 	"tailscale.com/types/views"
 )
 
@@ -111,7 +110,7 @@ func ApproveRoutesWithPolicy(pm PolicyManager, nv types.NodeView, currentApprove
 	}
 
 	// Sort and deduplicate
-	tsaddr.SortPrefixes(newApproved)
+	slices.SortFunc(newApproved, netip.Prefix.Compare)
 	newApproved = slices.Compact(newApproved)
 	newApproved = lo.Filter(newApproved, func(route netip.Prefix, index int) bool {
 		return route.IsValid()
@@ -120,7 +119,7 @@ func ApproveRoutesWithPolicy(pm PolicyManager, nv types.NodeView, currentApprove
 	// Sort the current approved for comparison
 	sortedCurrent := make([]netip.Prefix, len(currentApproved))
 	copy(sortedCurrent, currentApproved)
-	tsaddr.SortPrefixes(sortedCurrent)
+	slices.SortFunc(sortedCurrent, netip.Prefix.Compare)
 
 	// Only update if the routes actually changed
 	if !slices.Equal(sortedCurrent, newApproved) {