refactor(api): tighten fixed-session API contract

Require a single user message, reject mismatched models, document the OpenAI-compatible API, and exclude api/ from core agent line counts so the interface matches nanobot's minimal fixed-session runtime.

README.md

@@ -115,6 +115,7 @@
 - [Configuration](#️-configuration)
 - [Multiple Instances](#-multiple-instances)
 - [CLI Reference](#-cli-reference)
+- [OpenAI-Compatible API](#-openai-compatible-api)
 - [Docker](#-docker)
 - [Linux Service](#-linux-service)
 - [Project Structure](#-project-structure)
@@ -1541,6 +1542,7 @@ nanobot gateway --config ~/.nanobot-telegram/config.json --workspace /tmp/nanobo
 | `nanobot agent` | Interactive chat mode |
 | `nanobot agent --no-markdown` | Show plain-text replies |
 | `nanobot agent --logs` | Show runtime logs during chat |
+| `nanobot serve` | Start the OpenAI-compatible API |
 | `nanobot gateway` | Start the gateway |
 | `nanobot status` | Show status |
 | `nanobot provider login openai-codex` | OAuth login for providers |
@@ -1569,6 +1571,80 @@ The agent can also manage this file itself — ask it to "add a periodic task" a
 
 </details>
 
+## 🔌 OpenAI-Compatible API
+
+nanobot can expose a minimal OpenAI-compatible endpoint for local integrations:
+
+```bash
+pip install "nanobot-ai[api]"
+nanobot serve
+```
+
+By default, the API binds to `127.0.0.1:8900`.
+
+### Behavior
+
+- Fixed session: all requests share the same nanobot session (`api:default`)
+- Single-message input: each request must contain exactly one `user` message
+- Fixed model: omit `model`, or pass the same model shown by `/v1/models`
+- No streaming: `stream=true` is not supported
+
+### Endpoints
+
+- `GET /health`
+- `GET /v1/models`
+- `POST /v1/chat/completions`
+
+### curl
+
+```bash
+curl http://127.0.0.1:8900/v1/chat/completions \
+  -H "Content-Type: application/json" \
+  -d '{
+    "messages": [
+      {
+        "role": "user",
+        "content": "hi"
+      }
+    ]
+  }'
+```
+
+### Python (`requests`)
+
+```python
+import requests
+
+resp = requests.post(
+    "http://127.0.0.1:8900/v1/chat/completions",
+    json={
+        "messages": [
+            {"role": "user", "content": "hi"}
+        ]
+    },
+    timeout=120,
+)
+resp.raise_for_status()
+print(resp.json()["choices"][0]["message"]["content"])
+```
+
+### Python (`openai`)
+
+```python
+from openai import OpenAI
+
+client = OpenAI(
+    base_url="http://127.0.0.1:8900/v1",
+    api_key="dummy",
+)
+
+resp = client.chat.completions.create(
+    model="MiniMax-M2.7",
+    messages=[{"role": "user", "content": "hi"}],
+)
+print(resp.choices[0].message.content)
+```
+
 ## 🐳 Docker
 
 > [!TIP]

core_agent_lines.sh

@@ -1,5 +1,5 @@
 #!/bin/bash
-# Count core agent lines (excluding channels/, cli/, providers/ adapters)
+# Count core agent lines (excluding channels/, cli/, api/, providers/ adapters)
 cd "$(dirname "$0")" || exit 1
 
 echo "nanobot core agent line count"
@@ -15,7 +15,7 @@ root=$(cat nanobot/__init__.py nanobot/__main__.py | wc -l)
 printf "  %-16s %5s lines\n" "(root)" "$root"
 
 echo ""
-total=$(find nanobot -name "*.py" ! -path "*/channels/*" ! -path "*/cli/*" ! -path "*/command/*" ! -path "*/providers/*" ! -path "*/skills/*" | xargs cat | wc -l)
+total=$(find nanobot -name "*.py" ! -path "*/channels/*" ! -path "*/cli/*" ! -path "*/api/*" ! -path "*/command/*" ! -path "*/providers/*" ! -path "*/skills/*" | xargs cat | wc -l)
 echo "  Core total:     $total lines"
 echo ""
-echo "  (excludes: channels/, cli/, command/, providers/, skills/)"
+echo "  (excludes: channels/, cli/, api/, command/, providers/, skills/)"