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/mkall.sh

@@ -49,6 +49,7 @@ esac
 if [[ "$GOOS" = "linux" ]]; then
 	# Use the Docker-based build system
 	# Files generated through docker (use $cmd so you can Ctl-C the build or run)
+	set -e
 	$cmd docker build --tag generate:$GOOS $GOOS
 	$cmd docker run --interactive --tty --volume $(cd -- "$(dirname -- "$0")/.." && pwd):/build generate:$GOOS
 	exit