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/golang.org/x/sys/unix/auxv.go

@@ -0,0 +1,36 @@
+// Copyright 2025 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build go1.21 && (aix || darwin || dragonfly || freebsd || linux || netbsd || openbsd || solaris || zos)
+
+package unix
+
+import (
+	"syscall"
+	"unsafe"
+)
+
+//go:linkname runtime_getAuxv runtime.getAuxv
+func runtime_getAuxv() []uintptr
+
+// Auxv returns the ELF auxiliary vector as a sequence of key/value pairs.
+// The returned slice is always a fresh copy, owned by the caller.
+// It returns an error on non-ELF platforms, or if the auxiliary vector cannot be accessed,
+// which happens in some locked-down environments and build modes.
+func Auxv() ([][2]uintptr, error) {
+	vec := runtime_getAuxv()
+	vecLen := len(vec)
+
+	if vecLen == 0 {
+		return nil, syscall.ENOENT
+	}
+
+	if vecLen%2 != 0 {
+		return nil, syscall.EINVAL
+	}
+
+	result := make([]uintptr, vecLen)
+	copy(result, vec)
+	return unsafe.Slice((*[2]uintptr)(unsafe.Pointer(&result[0])), vecLen/2), nil
+}