Add agent explainability instrumentation and unify envelope field naming (#795)

Addresses recommendations from the UX developer's agent experience report.
Adds provenance predicates, DAG structure changes, error resilience, and
a published OWL ontology.

Explainability additions:

- Tool candidates: tg:toolCandidate on Analysis events lists the tools
  visible to the LLM for each iteration (names only, descriptions in config)
- Termination reason: tg:terminationReason on Conclusion/Synthesis events
  (final-answer, plan-complete, subagents-complete)
- Step counter: tg:stepNumber on iteration events
- Pattern decision: new tg:PatternDecision entity in the DAG between
  session and first iteration, carrying tg:pattern and tg:taskType
- Latency: tg:llmDurationMs on Analysis events, tg:toolDurationMs on
  Observation events
- Token counts on events: tg:inToken/tg:outToken/tg:llmModel on
  Grounding, Focus, Synthesis, and Analysis events
- Tool/parse errors: tg:toolError on Observation events with tg:Error
  mixin type. Parse failures return as error observations instead of
  crashing the agent, giving it a chance to retry.

Envelope unification:

- Rename chunk_type to message_type across AgentResponse schema,
  translator, SDK types, socket clients, CLI, and all tests.
  Agent and RAG services now both use message_type on the wire.

Ontology:

- specs/ontology/trustgraph.ttl — OWL vocabulary covering all 26 classes,
  7 object properties, and 36+ datatype properties including new predicates.

DAG structure tests:

- tests/unit/test_provenance/test_dag_structure.py verifies the
  wasDerivedFrom chain for GraphRAG, DocumentRAG, and all three agent
  patterns (react, plan, supervisor) including the pattern-decision link.

tests/unit/test_agent/test_callback_message_id.py

@@ -29,7 +29,7 @@ class TestThinkCallbackMessageId:
 
         assert len(responses) == 1
         assert responses[0].message_id == msg_id
-        assert responses[0].chunk_type == "thought"
+        assert responses[0].message_type == "thought"
 
     @pytest.mark.asyncio
     async def test_non_streaming_think_has_message_id(self, pattern):
@@ -58,7 +58,7 @@ class TestObserveCallbackMessageId:
         await observe("result", is_final=True)
 
         assert responses[0].message_id == msg_id
-        assert responses[0].chunk_type == "observation"
+        assert responses[0].message_type == "observation"
 
 
 class TestAnswerCallbackMessageId:
@@ -74,7 +74,7 @@ class TestAnswerCallbackMessageId:
         await answer("the answer")
 
         assert responses[0].message_id == msg_id
-        assert responses[0].chunk_type == "answer"
+        assert responses[0].message_type == "answer"
 
     @pytest.mark.asyncio
     async def test_no_message_id_default(self, pattern):