Wire message_id on all answer chunks, fix DAG structure (#748)

Wire message_id on all answer chunks, fix DAG structure message_id:
- Add message_id to AgentAnswer dataclass and propagate in
  socket_client._parse_chunk
- Wire message_id into answer callbacks and send_final_response
  for all three patterns (react, plan-then-execute, supervisor)
- Supervisor decomposition thought and synthesis answer chunks
  now carry message_id

DAG structure fixes:
- Observation derives from sub-trace Synthesis (not Analysis)
  when a tool produces a sub-trace; tracked via
  last_sub_explain_uri on context
- Subagent sessions derive from parent's Decomposition via
  parent_uri on agent_session_triples
- Findings derive from subagent Conclusions (not Decomposition)
- Synthesis derives from all findings (multiple wasDerivedFrom)
  ensuring single terminal node
- agent_synthesis_triples accepts list of parent URIs
- Explainability chain walker follows from sub-trace terminal
  to find downstream Observation

Emit Analysis before tool execution:
- Add on_action callback to react() in agent_manager.py, called
  after reason() but before tool invocation
- Orchestrator and old service emit Analysis+ToolUse triples via
  on_action so sub-traces appear after their parent in the stream

trustgraph-base/trustgraph/api/socket_client.py

@@ -397,7 +397,8 @@ class SocketClient:
             return AgentAnswer(
                 content=resp.get("content", ""),
                 end_of_message=resp.get("end_of_message", False),
-                end_of_dialog=resp.get("end_of_dialog", False)
+                end_of_dialog=resp.get("end_of_dialog", False),
+                message_id=resp.get("message_id", ""),
             )
         elif chunk_type == "action":
             return AgentThought(