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/os_other_arm64.go

@@ -0,0 +1,16 @@
+// Copyright (c) 2020 Klaus Post, released under MIT License. See LICENSE file.
+
+//go:build arm64 && !linux && !darwin
+// +build arm64,!linux,!darwin
+
+package cpuid
+
+import "runtime"
+
+func detectOS(c *CPUInfo) bool {
+	c.PhysicalCores = runtime.NumCPU()
+	// For now assuming 1 thread per core...
+	c.ThreadsPerCore = 1
+	c.LogicalCores = c.PhysicalCores
+	return false
+}