feat: add Python SDK facade and per-session isolation

docs/PYTHON_SDK.md

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+# Python SDK
+
+Use nanobot programmatically — load config, run the agent, get results.
+
+## Quick Start
+
+```python
+import asyncio
+from nanobot import Nanobot
+
+async def main():
+    bot = Nanobot.from_config()
+    result = await bot.run("What time is it in Tokyo?")
+    print(result.content)
+
+asyncio.run(main())
+```
+
+## API
+
+### `Nanobot.from_config(config_path?, *, workspace?)`
+
+Create a `Nanobot` from a config file.
+
+| Param | Type | Default | Description |
+|-------|------|---------|-------------|
+| `config_path` | `str \| Path \| None` | `None` | Path to `config.json`. Defaults to `~/.nanobot/config.json`. |
+| `workspace` | `str \| Path \| None` | `None` | Override workspace directory from config. |
+
+Raises `FileNotFoundError` if an explicit path doesn't exist.
+
+### `await bot.run(message, *, session_key?, hooks?)`
+
+Run the agent once. Returns a `RunResult`.
+
+| Param | Type | Default | Description |
+|-------|------|---------|-------------|
+| `message` | `str` | *(required)* | The user message to process. |
+| `session_key` | `str` | `"sdk:default"` | Session identifier for conversation isolation. Different keys get independent history. |
+| `hooks` | `list[AgentHook] \| None` | `None` | Lifecycle hooks for this run only. |
+
+```python
+# Isolated sessions — each user gets independent conversation history
+await bot.run("hi", session_key="user-alice")
+await bot.run("hi", session_key="user-bob")
+```
+
+### `RunResult`
+
+| Field | Type | Description |
+|-------|------|-------------|
+| `content` | `str` | The agent's final text response. |
+| `tools_used` | `list[str]` | Tool names invoked during the run. |
+| `messages` | `list[dict]` | Raw message history (for debugging). |
+
+## Hooks
+
+Hooks let you observe or modify the agent loop without touching internals.
+
+Subclass `AgentHook` and override any method:
+
+| Method | When |
+|--------|------|
+| `before_iteration(ctx)` | Before each LLM call |
+| `on_stream(ctx, delta)` | On each streamed token |
+| `on_stream_end(ctx)` | When streaming finishes |
+| `before_execute_tools(ctx)` | Before tool execution (inspect `ctx.tool_calls`) |
+| `after_iteration(ctx, response)` | After each LLM response |
+| `finalize_content(ctx, content)` | Transform final output text |
+
+### Example: Audit Hook
+
+```python
+from nanobot.agent import AgentHook, AgentHookContext
+
+class AuditHook(AgentHook):
+    def __init__(self):
+        self.calls = []
+
+    async def before_execute_tools(self, ctx: AgentHookContext) -> None:
+        for tc in ctx.tool_calls:
+            self.calls.append(tc.name)
+            print(f"[audit] {tc.name}({tc.arguments})")
+
+hook = AuditHook()
+result = await bot.run("List files in /tmp", hooks=[hook])
+print(f"Tools used: {hook.calls}")
+```
+
+### Composing Hooks
+
+Pass multiple hooks — they run in order, errors in one don't block others:
+
+```python
+result = await bot.run("hi", hooks=[AuditHook(), MetricsHook()])
+```
+
+Under the hood this uses `CompositeHook` for fan-out with error isolation.
+
+### `finalize_content` Pipeline
+
+Unlike the async methods (fan-out), `finalize_content` is a pipeline — each hook's output feeds the next:
+
+```python
+class Censor(AgentHook):
+    def finalize_content(self, ctx, content):
+        return content.replace("secret", "***") if content else content
+```
+
+## Full Example
+
+```python
+import asyncio
+from nanobot import Nanobot
+from nanobot.agent import AgentHook, AgentHookContext
+
+class TimingHook(AgentHook):
+    async def before_iteration(self, ctx: AgentHookContext) -> None:
+        import time
+        ctx.metadata["_t0"] = time.time()
+
+    async def after_iteration(self, ctx, response) -> None:
+        import time
+        elapsed = time.time() - ctx.metadata.get("_t0", 0)
+        print(f"[timing] iteration took {elapsed:.2f}s")
+
+async def main():
+    bot = Nanobot.from_config(workspace="/my/project")
+    result = await bot.run(
+        "Explain the main function",
+        hooks=[TimingHook()],
+    )
+    print(result.content)
+
+asyncio.run(main())
+```