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_gateway/test_explain_triples.py

@@ -158,7 +158,7 @@ class TestAgentExplainTriples:
         translator = AgentResponseTranslator()
 
         response = AgentResponse(
-            chunk_type="explain",
+            message_type="explain",
             content="",
             explain_id="urn:trustgraph:agent:session:abc123",
             explain_graph="urn:graph:retrieval",
@@ -179,7 +179,7 @@ class TestAgentExplainTriples:
         translator = AgentResponseTranslator()
 
         response = AgentResponse(
-            chunk_type="thought",
+            message_type="thought",
             content="I need to think...",
         )
 
@@ -190,7 +190,7 @@ class TestAgentExplainTriples:
         translator = AgentResponseTranslator()
 
         response = AgentResponse(
-            chunk_type="explain",
+            message_type="explain",
             explain_id="urn:trustgraph:agent:session:abc123",
             explain_triples=sample_triples(),
             end_of_dialog=False,
@@ -203,7 +203,7 @@ class TestAgentExplainTriples:
         translator = AgentResponseTranslator()
 
         response = AgentResponse(
-            chunk_type="answer",
+            message_type="answer",
             content="The answer is...",
             end_of_dialog=True,
         )