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/Makefile

@@ -4,7 +4,7 @@ export CGO_ENABLED := 1
 
 .PHONY: test
 test:
-	go test ./...
+	go test -race -timeout 15s ./...
 
 .PHONY: bench
 bench:
@@ -31,10 +31,6 @@ build-nocgo:
 # Run cross-compilation to assure supported architectures.
 cross-build: build-arm build-arm64 build-nocgo
 
-generate:
-	go run sha1cdblock_amd64_asm.go -out sha1cdblock_amd64.s
-	sed -i 's;&\samd64;&\n// +build !noasm,gc,amd64;g' sha1cdblock_amd64.s
-
-verify: generate
+verify:
 	git diff --exit-code
 	go vet ./...