feat: add cmd/goroutinectx for standalone CLI usage

Enables:
- go install github.com/mpyw/goroutinectx/cmd/goroutinectx@latest
- go vet -vettool=$(which goroutinectx) ./...

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: mpyw

cmd/goroutinectx/main.go

@@ -0,0 +1,11 @@
+// Command goroutinectx is a linter that checks goroutine context propagation.
+package main
+
+import (
+	"github.com/mpyw/goroutinectx"
+	"golang.org/x/tools/go/analysis/singlechecker"
+)
+
+func main() {
+	singlechecker.Main(goroutinectx.Analyzer)
+}