refactor: link internal/buildcfg.ToolTags and enhance demo test coverage

- Link internal/buildcfg.ToolTags variable instead of hardcoding tool tags
- Add comprehensive test cases in demo.go to verify:
  - build.Default.Compiler is correctly patched to "gc"
  - ToolTags are properly populated from internal/buildcfg
  - ReleaseTags are correctly generated
- Validates the go/build patches work as expected

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

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

_demo/go/gobuild/demo.go

@@ -10,6 +10,23 @@ import (
 
 func main() {
 	fmt.Printf("runtime.Compiler = %q\n", runtime.Compiler)
+
+	ctx := build.Default
+	fmt.Printf("build.Default.Compiler = %q\n", ctx.Compiler)
+	if ctx.Compiler != "gc" {
+		panic(fmt.Sprintf("expected build.Default.Compiler to be \"gc\", got %q", ctx.Compiler))
+	}
+
+	if len(ctx.ToolTags) == 0 {
+		panic("expected build.Default.ToolTags to be non-empty")
+	}
+	fmt.Printf("build.Default.ToolTags = %v\n", ctx.ToolTags)
+
+	if len(ctx.ReleaseTags) == 0 {
+		panic("expected build.Default.ReleaseTags to be non-empty")
+	}
+	fmt.Printf("build.Default.ReleaseTags count = %d\n", len(ctx.ReleaseTags))
+
 	pkg, err := build.Import("fmt", "", build.FindOnly)
 	if err != nil {
 		panic(err)