refactor: organize agent harness courses

learn-claude-code/docs/en/s08-background-tasks.md

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+# s08: Background Tasks
+
+`s01 > s02 > s03 > s04 > s05 > s06 | s07 > [ s08 ] s09 > s10 > s11 > s12`
+
+> *"Run slow operations in the background; the agent keeps thinking"* -- daemon threads run commands, inject notifications on completion.
+>
+> **Harness layer**: Background execution -- the model thinks while the harness waits.
+
+## Problem
+
+Some commands take minutes: `npm install`, `pytest`, `docker build`. With a blocking loop, the model sits idle waiting. If the user asks "install dependencies and while that runs, create the config file," the agent does them sequentially, not in parallel.
+
+## Solution
+
+```
+Main thread                Background thread
++-----------------+        +-----------------+
+| agent loop      |        | subprocess runs |
+| ...             |        | ...             |
+| [LLM call] <---+------- | enqueue(result) |
+|  ^drain queue   |        +-----------------+
++-----------------+
+
+Timeline:
+Agent --[spawn A]--[spawn B]--[other work]----
+             |          |
+             v          v
+          [A runs]   [B runs]      (parallel)
+             |          |
+             +-- results injected before next LLM call --+
+```
+
+## How It Works
+
+1. BackgroundManager tracks tasks with a thread-safe notification queue.
+
+```python
+class BackgroundManager:
+    def __init__(self):
+        self.tasks = {}
+        self._notification_queue = []
+        self._lock = threading.Lock()
+```
+
+2. `run()` starts a daemon thread and returns immediately.
+
+```python
+def run(self, command: str) -> str:
+    task_id = str(uuid.uuid4())[:8]
+    self.tasks[task_id] = {"status": "running", "command": command}
+    thread = threading.Thread(
+        target=self._execute, args=(task_id, command), daemon=True)
+    thread.start()
+    return f"Background task {task_id} started"
+```
+
+3. When the subprocess finishes, its result goes into the notification queue.
+
+```python
+def _execute(self, task_id, command):
+    try:
+        r = subprocess.run(command, shell=True, cwd=WORKDIR,
+            capture_output=True, text=True, timeout=300)
+        output = (r.stdout + r.stderr).strip()[:50000]
+    except subprocess.TimeoutExpired:
+        output = "Error: Timeout (300s)"
+    with self._lock:
+        self._notification_queue.append({
+            "task_id": task_id, "result": output[:500]})
+```
+
+4. The agent loop drains notifications before each LLM call.
+
+```python
+def agent_loop(messages: list):
+    while True:
+        notifs = BG.drain_notifications()
+        if notifs:
+            notif_text = "\n".join(
+                f"[bg:{n['task_id']}] {n['result']}" for n in notifs)
+            messages.append({"role": "user",
+                "content": f"<background-results>\n{notif_text}\n"
+                           f"</background-results>"})
+        response = client.messages.create(...)
+```
+
+The loop stays single-threaded. Only subprocess I/O is parallelized.
+
+## What Changed From s07
+
+| Component      | Before (s07)     | After (s08)                |
+|----------------|------------------|----------------------------|
+| Tools          | 8                | 6 (base + background_run + check)|
+| Execution      | Blocking only    | Blocking + background threads|
+| Notification   | None             | Queue drained per loop     |
+| Concurrency    | None             | Daemon threads             |
+
+## Try It
+
+```sh
+cd learn-claude-code
+python agents/s08_background_tasks.py
+```
+
+1. `Run "sleep 5 && echo done" in the background, then create a file while it runs`
+2. `Start 3 background tasks: "sleep 2", "sleep 4", "sleep 6". Check their status.`
+3. `Run pytest in the background and keep working on other things`