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/detect_ref.go

@@ -0,0 +1,17 @@
+// Copyright (c) 2015 Klaus Post, released under MIT License. See LICENSE file.
+
+//go:build (!amd64 && !386 && !arm64) || gccgo || noasm || appengine
+// +build !amd64,!386,!arm64 gccgo noasm appengine
+
+package cpuid
+
+func initCPU() {
+	cpuid = func(uint32) (a, b, c, d uint32) { return 0, 0, 0, 0 }
+	cpuidex = func(x, y uint32) (a, b, c, d uint32) { return 0, 0, 0, 0 }
+	xgetbv = func(uint32) (a, b uint32) { return 0, 0 }
+	rdtscpAsm = func() (a, b, c, d uint32) { return 0, 0, 0, 0 }
+
+}
+
+func addInfo(info *CPUInfo, safe bool) {}
+func getVectorLength() (vl, pl uint64) { return 0, 0 }