feat(audit): read-only knowledge-graph drift report + skill

Adds audit_knowledge_graph (MCP tool) and GET /api/v1/audit/drift (endpoint)
backed by a shared internal/audit package. One pass surfaces the structural
gaps an operator otherwise finds by accident: orphan check entities, checks
targeting deprecated/destroyed entities, probes stuck down/unknown, unmonitored
declared types, and live edges pointing at destroyed targets. Each finding
carries a suggested remediation runbook. Read-only and safe to run unattended.

Ships the knowledge-graph-audit skill (SKILL.md + seeded runbook) that
interprets the report and routes findings to the lifecycle runbooks.

internal/audit/testutil_test.go

@@ -0,0 +1,67 @@
+package audit
+
+import (
+	"context"
+	"fmt"
+	"math/rand"
+	"os"
+	"strings"
+	"testing"
+
+	"github.com/dtoro/oikos/internal/db"
+	"github.com/jackc/pgx/v5"
+)
+
+// createTestDB provisions a throwaway migrated database, same convention as
+// internal/scheduler/coverage_test.go.
+func createTestDB(t *testing.T, baseURL string) *db.Pool {
+	t.Helper()
+	ctx := context.Background()
+
+	admin, err := pgx.Connect(ctx, baseURL)
+	if err != nil {
+		t.Fatalf("connect admin: %v", err)
+	}
+	dbName := fmt.Sprintf("oikos_aud_%08x", rand.Int63())
+	if _, err := admin.Exec(ctx, "CREATE DATABASE "+dbName); err != nil {
+		admin.Close(ctx)
+		t.Fatalf("create test db: %v", err)
+	}
+	admin.Close(ctx)
+
+	at := strings.LastIndex(baseURL, "/")
+	testURL := baseURL[:at+1] + dbName
+	if q := strings.Index(baseURL[at:], "?"); q >= 0 {
+		testURL += baseURL[at+q:]
+	}
+
+	pool, err := db.New(ctx, testURL)
+	if err != nil {
+		t.Fatalf("connect test db: %v", err)
+	}
+	if err := pool.Migrate(ctx); err != nil {
+		t.Fatalf("migrate: %v", err)
+	}
+	t.Cleanup(func() {
+		pool.Close()
+		if admin, err := pgx.Connect(ctx, baseURL); err == nil {
+			admin.Exec(ctx, "DROP DATABASE IF EXISTS "+dbName+" WITH (FORCE)")
+			admin.Close(ctx)
+		}
+	})
+	return pool
+}
+
+func getenvOrDefault(key, def string) string {
+	if v := os.Getenv(key); v != "" {
+		return v
+	}
+	return def
+}
+
+func mustExec(t *testing.T, pool *db.Pool, ctx context.Context, q string, args ...any) {
+	t.Helper()
+	if _, err := pool.Exec(ctx, q, args...); err != nil {
+		t.Fatalf("exec %s: %v", q, err)
+	}
+}