feat: consolidate course into 21 lessons

s12_task_system/code.py

@@ -14,10 +14,8 @@ Changes from s11:
   - complete_task: set completed + report unblocked downstream
   - 5 new tools: create_task, list_tasks, get_task, claim_task, complete_task
 
-Note: Teaching code keeps a basic agent loop to stay focused on the task
-system. S11's full error recovery (RecoveryState, backoff, escalation,
-reactive compact, fallback model) is omitted — in real CC, tasks.ts and
-withRetry are independent layers that compose naturally.
+This chapter keeps the agent loop focused on the task system. Error recovery
+remains the independent layer introduced in s11.
 """
 
 import os, subprocess, json, time, random