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/service-routing.ts

@@ -5,7 +5,6 @@ import {
   CODEX_REVIEW_SERVICE_ID,
   OWNER_AGENT_TYPE,
   REVIEWER_SERVICE_ID_FOR_TYPE,
-  SERVICE_AGENT_TYPE,
   SERVICE_ID,
   isArbiterEnabled,
   normalizeServiceId,
@@ -99,9 +98,7 @@ function getDefaultLease(chatJid: string): EffectiveChannelLease {
   return {
     chat_jid: chatJid,
     owner_service_id:
-      SERVICE_AGENT_TYPE === 'codex'
-        ? CODEX_MAIN_SERVICE_ID
-        : CLAUDE_SERVICE_ID,
+      CLAUDE_SERVICE_ID,
     reviewer_service_id: null,
     arbiter_service_id: null,
     activated_at: null,