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/pjbgf/sha1cd/sha1cdblock_noasm.go

@@ -1,5 +1,4 @@
-//go:build !amd64 || noasm || !gc
-// +build !amd64 noasm !gc
+//go:build (!amd64 && !arm64) || noasm
 
 package sha1cd