fix: restore README.md for Python SDK package build

The README.md file is required by pyproject.toml for package metadata.
Restored from commit 61bf242.

Author: RoyLin

sdk/python/README.md

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+# A3S Code — Python SDK
+
+Native Python bindings for the A3S Code AI coding agent, built with PyO3.
+
+## Installation
+
+```bash
+pip install a3s-code
+```
+
+## Quick Start
+
+```python
+from a3s_code import Agent
+
+agent = Agent.create("agent.hcl")
+session = agent.session("/my-project")
+
+result = session.send("What files handle authentication?")
+print(result.text)
+```
+
+## Slash Commands
+
+Every session includes built-in slash commands dispatched before the LLM:
+
+```python
+# List all available commands
+commands = session.list_commands()
+for cmd in commands:
+    print(f"/{cmd['name']:15s} {cmd['description']}")
+
+# Built-in commands
+result = session.send("/help")       # List all commands
+result = session.send("/model")      # Show current model
+result = session.send("/cost")       # Token usage and cost
+result = session.send("/history")    # Conversation stats
+result = session.send("/cron-list")  # List scheduled tasks
+```
+
+### Custom Commands
+
+```python
+def my_handler(args: str, ctx: dict) -> str:
+    return f"Model: {ctx['model']}, History: {ctx['history_len']} msgs, args: {args!r}"
+
+session.register_command("status", "Show session info", my_handler)
+result = session.send("/status hello")
+```
+
+## Scheduled Tasks
+
+Schedule recurring prompts that fire after each `send()` call:
+
+```python
+# Via /loop slash command
+r = session.send("/loop 30s check deployment status")
+print(r.text)  # Scheduled [a1b2c3d4]: "check deployment status" — fires every 30s
+
+# Programmatic API
+task_id = session.schedule_task("summarize recent commits", 300)  # every 5 min
+
+# List active tasks
+for t in session.list_scheduled_tasks():
+    print(f"[{t['id']}] every {t['interval_secs']}s — \"{t['prompt']}\"")
+
+# Cancel
+session.cancel_scheduled_task(task_id)
+session.send(f"/cron-cancel {task_id}")
+```
+
+**Interval syntax:** `30s`, `5m`, `2h`, `1d`. Leading or trailing with `every` clause.
+
+## Full API
+
+```python
+from a3s_code import Agent, SessionOptions, DefaultSecurityProvider, FileMemoryStore, FileSessionStore
+
+agent = Agent.create("agent.hcl")
+session = agent.session("/my-project",
+    model="openai/gpt-4o",
+    builtin_skills=True,
+    planning=True,
+)
+
+# Send / Stream
+result = session.send("Explain the auth module")
+for event in session.stream("Refactor auth"):
+    if event.event_type == "text_delta":
+        print(event.text, end="", flush=True)
+
+# Direct tools (bypass LLM)
+session.read_file("src/main.py")
+session.bash("pytest")
+session.glob("**/*.py")
+session.grep("TODO")
+
+# Slash commands & scheduling
+session.list_commands()
+session.register_command("ping", "Pong!", lambda args, ctx: "pong")
+task_id = session.schedule_task("daily report", 86400)
+session.list_scheduled_tasks()
+session.cancel_scheduled_task(task_id)
+
+# Memory
+session.remember_success("task", ["tool"], "result")
+session.recall_similar("auth", 5)
+
+# Hooks
+session.register_hook("audit", "pre_tool_use", handler_fn)
+
+# MCP
+session.add_mcp_server("github", command="npx", args=["-y", "@modelcontextprotocol/server-github"])
+session.mcp_status()
+session.tool_names()
+session.remove_mcp_server("github")
+
+# Persistence
+opts = SessionOptions()
+opts.session_store = FileSessionStore('./sessions')
+opts.session_id = 'my-session'
+opts.auto_save = True
+session2 = agent.session(".", opts)
+resumed = agent.resume_session('my-session', opts)
+```
+
+## License
+
+MIT