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/platform-prompts.ts

@@ -63,13 +63,8 @@ export function readPairedRoomPrompt(
   return prompt || undefined;
 }
 
-export function getArbiterPromptPath(
-  projectRoot = process.cwd(),
-): string {
-  return path.join(
-    getPlatformPromptsDir(projectRoot),
-    ARBITER_PROMPT_FILE,
-  );
+export function getArbiterPromptPath(projectRoot = process.cwd()): string {
+  return path.join(getPlatformPromptsDir(projectRoot), ARBITER_PROMPT_FILE);
 }
 
 export function readArbiterPrompt(