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

.pre-commit-config.yaml

@@ -43,26 +43,12 @@ repos:
         entry: prettier --write --list-different
         language: system
         exclude: ^docs/
-        types_or:
-          [
-            javascript,
-            jsx,
-            ts,
-            tsx,
-            yaml,
-            json,
-            toml,
-            html,
-            css,
-            scss,
-            sass,
-            markdown,
-          ]
+        types_or: [javascript, jsx, ts, tsx, yaml, json, toml, html, css, scss, sass, markdown]
 
       # golangci-lint for Go code quality
       - id: golangci-lint
         name: golangci-lint
-        entry: nix develop --command golangci-lint run --new-from-rev=HEAD~1 --timeout=5m --fix
+        entry: golangci-lint run --new-from-rev=HEAD~1 --timeout=5m --fix
         language: system
         types: [go]
         pass_filenames: false