refactor: remove SERVICE_AGENT_TYPE legacy constant

SERVICE_AGENT_TYPE was always 'claude-code' in the unified service,
making it a misleading constant. Each group already has its own
agentType field.

- Removed SERVICE_AGENT_TYPE from config.ts
- Session functions now accept agentType parameter (default: 'claude-code')
- Task scheduler uses task-level agent_type for token rotation decisions
- All fallback defaults changed to 'claude-code' literal
- Logging uses 'unified' instead of the misleading type

src/message-runtime.ts

@@ -26,7 +26,6 @@ import {
   CODEX_REVIEW_SERVICE_ID,
   isSessionCommandSenderAllowed,
   REVIEWER_AGENT_TYPE,
-  SERVICE_AGENT_TYPE,
   SERVICE_ID,
 } from './config.js';
 import { GroupQueue, GroupRunContext } from './group-queue.js';
@@ -75,7 +74,7 @@ export function isDuplicateOfLastBotFinal(
   }
 
   // Get the last bot final message from DB (any bot, not just this service)
-  const lastMessages = getLastBotFinalMessage(chatJid, SERVICE_AGENT_TYPE, 1);
+  const lastMessages = getLastBotFinalMessage(chatJid, 'claude-code', 1);
   if (lastMessages.length === 0) {
     return false;
   }