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/httpapi/server.go

@@ -237,6 +237,11 @@ func NewHandler(ctx context.Context, pool *db.Pool, cfg config.Config) http.Hand
 	r.With(combinedAuth(cfg, false)).Get("/api/v1/knowledge/orphans", s.serveKnowledgeOrphans)
 	r.With(combinedAuth(cfg, false)).Post("/api/v1/knowledge/merge", s.serveMergeKnowledge)
 
+	// Drift audit: read-only DB-side report of orphan checks, checks on
+	// retired targets, stuck down/unknown probes, unmonitored declared types,
+	// and dangling edges. Companion to the knowledge-graph-audit skill.
+	r.With(combinedAuth(cfg, false)).Get("/api/v1/audit/drift", s.serveAuditDrift)
+
 	// Custom (non-OpenAPI) routes: the global activity feed (recency-ordered,
 	// unlike ListExecutions which sorts by target for pagination) and the
 	// per-session "what did this session do" digest.