docs: refactor README into a docs-first landing page

docs/python-sdk.md

@@ -0,0 +1,31 @@
+# Python SDK
+
+Use nanobot as a library — no CLI, no gateway, just Python:
+
+```python
+from nanobot import Nanobot
+
+bot = Nanobot.from_config()
+result = await bot.run("Summarize the README")
+print(result.content)
+```
+
+Each call carries a `session_key` for conversation isolation — different keys get independent history:
+
+```python
+await bot.run("hi", session_key="user-alice")
+await bot.run("hi", session_key="task-42")
+```
+
+Add lifecycle hooks to observe or customize the agent:
+
+```python
+from nanobot.agent import AgentHook, AgentHookContext
+
+class AuditHook(AgentHook):
+    async def before_execute_tools(self, ctx: AgentHookContext) -> None:
+        for tc in ctx.tool_calls:
+            print(f"[tool] {tc.name}")
+
+result = await bot.run("Hello", hooks=[AuditHook()])
+```