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.test.ts

@@ -14,7 +14,6 @@ vi.mock('./agent-runner.js', () => ({
 vi.mock('./config.js', () => ({
   DATA_DIR: '/tmp/ejclaw-test-data',
   SERVICE_ID: 'claude',
-  SERVICE_AGENT_TYPE: 'claude-code',
   SERVICE_SESSION_SCOPE: 'claude',
   CODEX_MAIN_SERVICE_ID: 'codex-main',
   CODEX_REVIEW_SERVICE_ID: 'codex-review',