chore: modernize CI and update Go toolchain

- Bump Go from 1.19 to 1.26 and update all dependencies
- Rewrite CI workflow with matrix strategy (Linux, macOS, Windows)
- Update GitHub Actions to current versions (checkout@v4, setup-go@v5)
- Update CodeQL actions from v1 to v3
- Fix cross-platform bug in mock/path.go (path.Join -> filepath.Join)
- Clean up dependabot config (weekly schedule, remove stale ignore)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

vendor/github.com/klauspost/cpuid/v2/test-architectures.sh

@@ -0,0 +1,15 @@
+#!/bin/sh
+
+set -e
+
+go tool dist list | while IFS=/ read os arch; do
+    echo "Checking $os/$arch..."
+    echo " normal"
+    GOARCH=$arch GOOS=$os go build -o /dev/null .
+    echo " noasm"
+    GOARCH=$arch GOOS=$os go build -tags noasm -o /dev/null .
+    echo " appengine"
+    GOARCH=$arch GOOS=$os go build -tags appengine -o /dev/null .
+    echo " noasm,appengine"
+    GOARCH=$arch GOOS=$os go build -tags 'appengine noasm' -o /dev/null .
+done