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/cyphar/filepath-securejoin/internal/consts/consts.go

@@ -0,0 +1,15 @@
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Copyright (C) 2014-2015 Docker Inc & Go Authors. All rights reserved.
+// Copyright (C) 2017-2025 SUSE LLC. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package consts contains the definitions of internal constants used
+// throughout filepath-securejoin.
+package consts
+
+// MaxSymlinkLimit is the maximum number of symlinks that can be encountered
+// during a single lookup before returning -ELOOP. At time of writing, Linux
+// has an internal limit of 40.
+const MaxSymlinkLimit = 255