Split Analysis into Analysis+ToolUse and Observation, add message_id

Refactor agent provenance so that the decision (thought + tool
selection) and the result (observation) are separate DAG entities:

  Question ← Analysis+ToolUse ← Observation ← ... ← Conclusion

Analysis gains tg:ToolUse as a mixin RDF type and is emitted
before tool execution via an on_action callback in react().
This ensures sub-traces (e.g. GraphRAG) appear after their
parent Analysis in the streaming event order.

Observation becomes a standalone prov:Entity with tg:Observation
type, emitted after tool execution. The linear DAG chain runs
through Observation — subsequent iterations and the Conclusion
derive from it, not from the Analysis.

message_id is populated on streaming AgentResponse for thought
and observation chunks, using the provenance URI of the entity
being built. This lets clients group streamed chunks by entity.

Wire changes:
- provenance/agent.py: Add ToolUse type, new
  agent_observation_triples(), remove observation from iteration
- agent_manager.py: Add on_action callback between reason() and
  tool execution
- orchestrator/pattern_base.py: Split emit, wire message_id,
  chain through observation URIs
- orchestrator/react_pattern.py: Emit Analysis via on_action
  before tool runs
- agent/react/service.py: Same for non-orchestrator path
- api/explainability.py: New Observation class, updated dispatch
  and chain walker
- api/types.py: Add message_id to AgentThought/AgentObservation
- cli: Render Observation separately, [analysis: tool] labels

tests/unit/test_agent/test_provenance_triples.py

@@ -14,7 +14,7 @@ from trustgraph.provenance import (
 
 from trustgraph.provenance.namespaces import (
     RDF_TYPE, RDFS_LABEL,
-    PROV_ENTITY, PROV_WAS_DERIVED_FROM, PROV_WAS_GENERATED_BY,
+    PROV_ENTITY, PROV_WAS_DERIVED_FROM,
     TG_DECOMPOSITION, TG_FINDING, TG_PLAN_TYPE, TG_STEP_RESULT,
     TG_SYNTHESIS, TG_ANSWER_TYPE, TG_DOCUMENT,
     TG_SUBAGENT_GOAL, TG_PLAN_STEP,
@@ -63,7 +63,7 @@ class TestDecompositionTriples:
             "urn:decompose", "urn:session", ["goal-a"],
         )
         ts = _triple_set(triples)
-        assert ("urn:decompose", PROV_WAS_GENERATED_BY, "urn:session") in ts
+        assert ("urn:decompose", PROV_WAS_DERIVED_FROM, "urn:session") in ts
 
     def test_includes_goals(self):
         goals = ["What is X?", "What is Y?", "What is Z?"]
@@ -141,7 +141,7 @@ class TestPlanTriples:
             "urn:plan", "urn:session", ["step-a"],
         )
         ts = _triple_set(triples)
-        assert ("urn:plan", PROV_WAS_GENERATED_BY, "urn:session") in ts
+        assert ("urn:plan", PROV_WAS_DERIVED_FROM, "urn:session") in ts
 
     def test_includes_steps(self):
         steps = ["Define X", "Research Y", "Analyse Z"]