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/audit_test.go

@@ -0,0 +1,69 @@
+package audit
+
+import (
+	"context"
+	"testing"
+
+	"github.com/dtoro/oikos/internal/db"
+	"github.com/google/uuid"
+)
+
+// Integration tests against a real Postgres, guarded by
+// OIKOS_TEST_DATABASE_URL (same convention as internal/scheduler).
+
+func newAuditPool(t *testing.T) *db.Pool {
+	t.Helper()
+	base := getenvOrDefault("OIKOS_TEST_DATABASE_URL", "")
+	if base == "" {
+		t.Skip("OIKOS_TEST_DATABASE_URL not set — skipping integration test")
+	}
+	return createTestDB(t, base)
+}
+
+func TestReportFlagsOrphanAndDeadAndDown(t *testing.T) {
+	pool := newAuditPool(t)
+	ctx := context.Background()
+
+	// An orphan check entity (truncated random slug, the legacy bug shape).
+	mustExec(t, pool, ctx, `INSERT INTO entities (id, slug, type, name, state, attributes, version, created_at, updated_at)
+		VALUES ($1,'check:ssh-script:0d31fdd1','check','check:ssh-script:0d31fdd1','active','{}'::jsonb,1,now(),now())`, uuid.New())
+
+	// An active entity + a check_def on it stuck down.
+	target := uuid.New()
+	mustExec(t, pool, ctx, `INSERT INTO entities (id, slug, type, name, state, attributes, version, created_at, updated_at)
+		VALUES ($1,'service:demo','service','demo','active','{}'::jsonb,1,now(),now())`, target)
+	checkE := uuid.New()
+	mustExec(t, pool, ctx, `INSERT INTO entities (id, slug, type, name, state, attributes, version, created_at, updated_at)
+		VALUES ($1,'check:http:service:demo:0','check','c','active','{}'::jsonb,1,now(),now())`, checkE)
+	mustExec(t, pool, ctx, `INSERT INTO check_defs (entity_id, target_id, target_type, kind, config, interval_s, timeout_s, enabled, last_run_at, last_health)
+		VALUES ($1,$2,'service','http','{}'::jsonb,60,30,true,now(),'down')`, checkE, target)
+
+	// A deprecated entity still carrying an enabled check (dead_checks).
+	dep := uuid.New()
+	mustExec(t, pool, ctx, `INSERT INTO entities (id, slug, type, name, state, attributes, version, created_at, updated_at)
+		VALUES ($1,'service:old','service','old','deprecated','{}'::jsonb,1,now(),now())`, dep)
+	depCheck := uuid.New()
+	mustExec(t, pool, ctx, `INSERT INTO entities (id, slug, type, name, state, attributes, version, created_at, updated_at)
+		VALUES ($1,'check:http:service:old:0','check','c','active','{}'::jsonb,1,now(),now())`, depCheck)
+	mustExec(t, pool, ctx, `INSERT INTO check_defs (entity_id, target_id, target_type, kind, config, interval_s, timeout_s, enabled, last_run_at)
+		VALUES ($1,$2,'service','http','{}'::jsonb,60,30,true,now())`, depCheck, dep)
+
+	findings, summary := Report(ctx, pool)
+
+	byCat := map[string]int{}
+	for _, f := range findings {
+		byCat[f.Category] = f.Count
+	}
+	if byCat["orphan_checks"] < 1 {
+		t.Errorf("orphan_checks = %d, want >=1", byCat["orphan_checks"])
+	}
+	if byCat["down_checks"] < 1 {
+		t.Errorf("down_checks = %d, want >=1", byCat["down_checks"])
+	}
+	if byCat["dead_checks"] < 1 {
+		t.Errorf("dead_checks = %d, want >=1", byCat["dead_checks"])
+	}
+	if summary.TotalFindings < 3 {
+		t.Errorf("TotalFindings = %d, want >=3", summary.TotalFindings)
+	}
+}