refactor: rename sisyphus_task to delegate_task

- Rename directories: sisyphus-task → delegate-task
- Rename types: SisyphusTaskArgs → DelegateTaskArgs, etc.
- Rename functions: createSisyphusTask → createDelegateTask
- Rename constants: SISYPHUS_TASK_* → DELEGATE_TASK_*
- Update tool name: sisyphus_task → delegate_task
- Update all prompts, docs, and tests

src/features/background-agent/manager.test.ts

@@ -980,7 +980,7 @@ describe("BackgroundManager.trackTask", () => {
       sessionID: "session-1",
       parentSessionID: "parent-session",
       description: "external task",
-      agent: "sisyphus_task",
+      agent: "delegate_task",
       concurrencyKey: "external-key",
     }
 
@@ -1015,7 +1015,7 @@ describe("BackgroundManager.resume concurrency key", () => {
       sessionID: "session-1",
       parentSessionID: "parent-session",
       description: "external task",
-      agent: "sisyphus_task",
+      agent: "delegate_task",
       concurrencyKey: "external-key",
     })