File structure

minharness/
  __init__.py
  model.py          # the ONE place you plug in a real model (Ch 2)
  prompt.py         # PromptBuilder + structured-item helpers (Ch 3)
  context.py        # estimate_tokens, CostModel, Compactor (Ch 4, 5)
  tools.py          # ToolRegistry, shell tool, truncation (Ch 6)
  approvals.py      # ApprovalPolicy, gated_call, Checkpointer (Ch 7)
  parallel.py       # run_tools (Ch 8)
  memory.py         # MemoryLoader (Ch 9)
  skills.py         # SkillLoader + mini MCP (Ch 10)
  subagents.py      # spawn_subagent, orchestrate (Ch 11)
  control.py        # HookManager, Budget (Ch 12)
  agent.py          # the loop that wires it all together (Ch 2 + everything)
  main.py           # CLI entry point

Each file is essentially the MVP from its chapter, lightly cleaned so the imports line up. Below is the integration layer, agent.py, which is the new code: the glue that turns eleven separate demos into one agent.