fix: replace deprecated datetime.utcnow() with timezone-aware datetime.now(timezone.utc) (#816)

Python 3.14 deprecates datetime.utcnow(). Replace all 9 occurrences with
datetime.now(timezone.utc) and normalize the output to preserve the existing
ISO-8601 "Z"-suffixed format so downstream parsers are unaffected.

Fixes #814

trustgraph-flow/trustgraph/agent/orchestrator/pattern_base.py

@@ -9,7 +9,7 @@ librarian integration.
 import json
 import logging
 import uuid
-from datetime import datetime
+from datetime import datetime, timezone
 
 from ... schema import AgentRequest, AgentResponse, AgentStep, Error
 from ... schema import Triples, Metadata
@@ -253,7 +253,7 @@ class PatternBase:
                                    collection, respond, streaming,
                                    parent_uri=None):
         """Emit provenance triples for a new session."""
-        timestamp = datetime.utcnow().isoformat() + "Z"
+        timestamp = datetime.now(timezone.utc).isoformat().replace("+00:00", "Z")
         triples = set_graph(
             agent_session_triples(
                 session_uri, question, timestamp,

trustgraph-flow/trustgraph/agent/react/service.py

@@ -10,7 +10,7 @@ import sys
 import functools
 import logging
 import uuid
-from datetime import datetime
+from datetime import datetime, timezone
 
 # Module logger
 logger = logging.getLogger(__name__)
@@ -452,7 +452,7 @@ class Processor(AgentService):
 
             # On first iteration, emit session triples
             if iteration_num == 1:
-                timestamp = datetime.utcnow().isoformat() + "Z"
+                timestamp = datetime.now(timezone.utc).isoformat().replace("+00:00", "Z")
                 triples = set_graph(
                     agent_session_triples(session_uri, request.question, timestamp),
                     GRAPH_RETRIEVAL

trustgraph-flow/trustgraph/retrieval/document_rag/document_rag.py

@@ -2,7 +2,7 @@
 import asyncio
 import logging
 import uuid
-from datetime import datetime
+from datetime import datetime, timezone
 
 # Provenance imports
 from trustgraph.provenance import (
@@ -199,7 +199,7 @@ class DocumentRag:
         exp_uri = docrag_exploration_uri(session_id)
         syn_uri = docrag_synthesis_uri(session_id)
 
-        timestamp = datetime.utcnow().isoformat() + "Z"
+        timestamp = datetime.now(timezone.utc).isoformat().replace("+00:00", "Z")
 
         # Emit question explainability immediately
         if explain_callback:

trustgraph-flow/trustgraph/retrieval/graph_rag/graph_rag.py

@@ -7,7 +7,7 @@ import math
 import time
 import uuid
 from collections import OrderedDict
-from datetime import datetime
+from datetime import datetime, timezone
 
 from ... schema import Term, Triple as SchemaTriple, IRI, LITERAL, TRIPLE
 from ... knowledge import Uri, Literal
@@ -643,7 +643,7 @@ class GraphRag:
         foc_uri = make_focus_uri(session_id)
         syn_uri = make_synthesis_uri(session_id)
 
-        timestamp = datetime.utcnow().isoformat() + "Z"
+        timestamp = datetime.now(timezone.utc).isoformat().replace("+00:00", "Z")
 
         # Emit question explainability immediately
         if explain_callback: