feat: harden runtime readiness and session workflow

harness/tht/adapters/factory.py

@@ -27,7 +27,7 @@ def build_dwh(cfg: Config) -> DwhAdapter:
 
 
 def build_vector_store(cfg: Config, *, require_write: bool = False) -> VectorStore:
-    """Build the vector adapter, optionally requiring an HTTP writer credential."""
+    """Build the vector adapter, optionally requiring write capability."""
     resource = cfg.vectors
     if resource is None:
         raise ConfigError("Risorsa vectors non configurata")
@@ -35,11 +35,17 @@ def build_vector_store(cfg: Config, *, require_write: bool = False) -> VectorSto
     match resource.type:
         case "pgvector_direct":
             reader = resource.reader or resource.connection
-            if require_write and resource.writer is None:
+            # Legacy server workspaces use one RW `vector_db` connection. Keep
+            # that deployment contract without turning a workstation's legacy
+            # compatibility connection into an implicit writer.
+            writer = resource.writer or (
+                resource.connection if cfg.profile == "server" else None
+            )
+            if require_write and writer is None:
                 raise ConfigError("Vector writer non configurato per pgvector_direct")
             return PgVectorStore(
                 reader,
-                resource.writer,
+                writer,
                 expected_dimension=cfg.embeddings.dim if cfg.embeddings is not None else None,
             )
         case "thoth_vector_http":