feat: extract coverage-gate action from Cue for reuse across projects

- Move coveragegate tool from cue/tools to vociferate/coverage-gate
- Create composite action with JSON metrics output for CI
- Update tool to export passes/total_coverage/packages_checked/packages_failed
- Support per-package threshold policy via JSON configuration
- Change module path to git.hrafn.xyz/aether/vociferate/coverage-gate

coverage-gate/integration_test.go

@@ -0,0 +1,26 @@
+package main
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestRun_ExitCodeOnInvalidProfile(t *testing.T) {
+	t.Parallel()
+	tmp := t.TempDir()
+	policyPath := filepath.Join(tmp, "policy.json")
+	if err := os.WriteFile(policyPath, []byte(`{"minimum_statement_coverage":80,"critical_packages":[]}`), 0600); err != nil {
+		t.Fatalf("write policy: %v", err)
+	}
+
+	exit := run(
+		[]string{"--profile", filepath.Join(tmp, "missing.out"), "--policy", policyPath, "--src-root", "."},
+		os.Stdout,
+		os.Stderr,
+		func(_ string) ([]string, error) { return []string{"git.hrafn.xyz/aether/cue/service/llm"}, nil },
+	)
+	if exit != 1 {
+		t.Fatalf("expected exit 1 for missing profile, got %d", exit)
+	}
+}