Phase 7: Agent Selector — per-agent soul.md + ChromaDB memory filtering

- Agent dropdown UI (chip button + hidden select) in composer header
- Session.agent field persists agent selection across refresh
- soul.md loaded per-agent via ephemeral_system_prompt injection
- ChromaDB memory filtered by agent topic (lotus/, sunflower/, etc.)
- Fixed streaming.py: agent→_ai_agent variable shadowing (lines 1161, 1163)
- New API endpoints: /api/agents/topology, /api/agents/memory/search
- Agent metadata registry with emoji, name, description per Tier-2 agent

static/messages.js

@@ -71,7 +71,9 @@ async function send(){
   try{
     const startData=await api('/api/chat/start',{method:'POST',body:JSON.stringify({
       session_id:activeSid,message:msgText,
-      model:S.session.model||$('modelSelect').value,workspace:S.session.workspace,
+      model:S.session.model||$('modelSelect').value,
+      agent:S.session.agent||$('agentSelect').value,
+      workspace:S.session.workspace,
       attachments:uploaded.length?uploaded:undefined
     })});
     streamId=startData.stream_id;