feat: Phase 4 — RelationshipService, postgres RelRepo, converged edges

Problem: relationship create/end existed as three drifted copies
(HTTP CreateRelationship/EndRelationship, MCP create_relationship/
end_relationship) with inline SQL, no ontology edge validation on
either path, and no audit on the MCP path.

Change:
- Internal/adapters/postgres/repositories.go: RelRepo implements
  ports.RelationshipRepository (Create/End/ListFor) over the pool,
  with in-tx upsert + audit/event side effects on Create.
- Internal/core/app/relationships.go: RelationshipService validates
  edges against the cached ontology TypeTree (tree.ValidateEdge) and
  delegates the tx to the repository. The adapter resolves slug→entity
  and extracts types before calling the service.
- HTTP CreateRelationship: resolves source/target via ReadModels,
  passes resolved types to RelationshipService for edge validation.
  EndRelationship calls the service directly (audit stays in the
  adapter for End — a simple toggle with no ontology check).
- MCP create_relationship/end_relationship: rewired to the service
  (pool resolves entity IDs inline for the tool handlers; the service
  validates edges and writes audit). The MCP path now gets ontology
  validation and audit coverage for the first time.
- Composition root: RelationshipService built with RelRepo + Ontology
  and wired through httpapi.NewHandler, ListenAndServe, and MCP
  constructors.

Verification: go build/vet, full test suite (19 pkgs, DB integration
postgres+mcp green).

internal/httpapi/server.go

@@ -65,9 +65,10 @@ type Server struct {
 	// entities is the entity-aggregate use-case service (Phase 3 of the
 	// hexagonal refactor); entityRepo exposes the idempotency reads the
 	// replay path needs. Wired here until main becomes the composition root.
-	entities   *app.EntityService
-	entityRepo *db.EntityRepo
-	readModels ports.ReadModels
+	entities    *app.EntityService
+	entityRepo  *db.EntityRepo
+	readModels  ports.ReadModels
+	relService  *app.RelationshipService
 }
 
 // NewHandler builds the full HTTP handler: /healthz (unauthenticated,
@@ -77,7 +78,7 @@ type Server struct {
 // holds a dedicated pooled connection for LISTEN. Callers MUST cancel ctx
 // before closing the pool — otherwise the held connection never releases
 // and pool.Close() deadlocks.
-func NewHandler(ctx context.Context, pool *db.Pool, cfg config.Config, entities *app.EntityService, entityRepo *db.EntityRepo, readModels ports.ReadModels) http.Handler {
+func NewHandler(ctx context.Context, pool *db.Pool, cfg config.Config, entities *app.EntityService, entityRepo *db.EntityRepo, readModels ports.ReadModels, relService *app.RelationshipService) http.Handler {
 	s := &Server{
 		pool:        pool,
 		cfg:         cfg,
@@ -87,6 +88,7 @@ func NewHandler(ctx context.Context, pool *db.Pool, cfg config.Config, entities
 		entities:    entities,
 		entityRepo:  entityRepo,
 		readModels:  readModels,
+		relService:  relService,
 	}
 
 	// Wire secrets backend: Infisical primary with SOPS DR fallback.
@@ -330,7 +332,7 @@ func NewHandler(ctx context.Context, pool *db.Pool, cfg config.Config, entities
 	if nomosAgentID == uuid.Nil && cfg.NomosAgentSlug != "" {
 		_ = pool.QueryRow(ctx, "SELECT id FROM entities WHERE slug = $1", cfg.NomosAgentSlug).Scan(&nomosAgentID)
 	}
-	r.With(combinedAuth(cfg, false)).Handle("/mcp", mcphandler.NewHandler(pool, cfg.MCPBearerToken, nomosAgentID, s.secretsManager, s.entities))
+	r.With(combinedAuth(cfg, false)).Handle("/mcp", mcphandler.NewHandler(pool, cfg.MCPBearerToken, nomosAgentID, s.secretsManager, s.entities, s.relService))
 
 	if nomosURL := os.Getenv("NOMOS_PROXY_URL"); nomosURL != "" {
 		target, _ := url.Parse(nomosURL)
@@ -935,10 +937,10 @@ main();
 
 // ListenAndServe runs the API server with graceful shutdown on ctx cancel
 // (SG4): stop accepting, drain in-flight for up to 30s, then exit.
-func ListenAndServe(ctx context.Context, pool *db.Pool, cfg config.Config, entities *app.EntityService, entityRepo *db.EntityRepo, readModels ports.ReadModels) error {
+func ListenAndServe(ctx context.Context, pool *db.Pool, cfg config.Config, entities *app.EntityService, entityRepo *db.EntityRepo, readModels ports.ReadModels, relService *app.RelationshipService) error {
 	srv := &http.Server{
 		Addr:              cfg.APIListen,
-		Handler:           NewHandler(ctx, pool, cfg, entities, entityRepo, readModels),
+		Handler:           NewHandler(ctx, pool, cfg, entities, entityRepo, readModels, relService),
 		ReadHeaderTimeout: 10 * time.Second,
 	}