feat: add Atomic Chat as OpenAI-compatible local provider

Register atomic_chat in the provider registry with default base URL
http://localhost:1337/v1, schema field, docs, and config tests.

docs/configuration.md

@@ -80,6 +80,7 @@ IMAP_PASSWORD=your-password-here
 | `longcat` | LLM (LongCat) | [longcat.chat](https://longcat.chat/platform/docs/zh/) |
 | `ollama` | LLM (local, Ollama) | — |
 | `lm_studio` | LLM (local, LM Studio) | — |
+| `atomic_chat` | LLM (local, [Atomic Chat](https://atomic.chat/)) | — |
 | `mistral` | LLM | [docs.mistral.ai](https://docs.mistral.ai/) |
 | `stepfun` | LLM (Step Fun/阶跃星辰) | [platform.stepfun.com](https://platform.stepfun.com) |
 | `ovms` | LLM (local, OpenVINO Model Server) | [docs.openvino.ai](https://docs.openvino.ai/2026/model-server/ovms_docs_llm_quickstart.html) |
@@ -502,6 +503,36 @@ ollama run llama3.2
 
 </details>
 
+<details>
+<summary><b>Atomic Chat (local)</b></summary>
+
+[Atomic Chat](https://atomic.chat/) is a local-first desktop app that exposes an **OpenAI-compatible** HTTP API (default `http://localhost:1337/v1`). Start Atomic Chat and enable the local API server, then point nanobot at it.
+
+**1. Add to config** (partial — merge into `~/.nanobot/config.json`):
+
+```json
+{
+  "providers": {
+    "atomic_chat": {
+      "apiKey": null,
+      "apiBase": "http://localhost:1337/v1"
+    }
+  },
+  "agents": {
+    "defaults": {
+      "provider": "atomic_chat",
+      "model": "your-model-id-from-atomic-chat"
+    }
+  }
+}
+```
+
+> **Note:** Set `apiKey` to `null` if your Atomic Chat server does not require a key. If it does, set `apiKey` (or the `ATOMIC_CHAT_API_KEY` environment variable) to the value Atomic Chat expects. The `model` string must match the model id Atomic Chat exposes on its OpenAI-compatible endpoint.
+
+> `provider: "auto"` also works when `providers.atomic_chat.apiBase` is configured, but setting `"provider": "atomic_chat"` is the clearest option.
+
+</details>
+
 <details>
 <summary><b>OpenVINO Model Server (local / OpenAI-compatible)</b></summary>