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/routes/primary.go

@@ -240,7 +240,7 @@ func (pr *PrimaryRoutes) PrimaryRoutes(id types.NodeID) []netip.Prefix {
 		}
 	}
 
-	tsaddr.SortPrefixes(routes)
+	slices.SortFunc(routes, netip.Prefix.Compare)
 
 	return routes
 }
@@ -300,7 +300,7 @@ func (pr *PrimaryRoutes) DebugJSON() DebugRoutes {
 	// Populate available routes
 	for nodeID, routes := range pr.routes {
 		prefixes := routes.Slice()
-		tsaddr.SortPrefixes(prefixes)
+		slices.SortFunc(prefixes, netip.Prefix.Compare)
 		debug.AvailableRoutes[nodeID] = prefixes
 	}