feat(gateway): add OpenAI-compatible HTTP endpoint

docs/gateway/openai-http-api.md

@@ -0,0 +1,72 @@
+---
+summary: "Expose an OpenAI-compatible /v1/chat/completions HTTP endpoint from the Gateway"
+read_when:
+  - Integrating tools that expect OpenAI Chat Completions
+---
+# OpenAI-compatible HTTP API
+
+Clawdbot’s Gateway can serve a small OpenAI-compatible endpoint:
+
+- `POST /v1/chat/completions`
+- Same port as the Gateway (WS + HTTP multiplex): `http://<gateway-host>:<port>/v1/chat/completions`
+
+## Authentication
+
+Uses the Gateway auth configuration. Send a bearer token:
+
+- `Authorization: Bearer <token>`
+
+Notes:
+- When `gateway.auth.mode="token"`, use `gateway.auth.token` (or `CLAWDBOT_GATEWAY_TOKEN`).
+- When `gateway.auth.mode="password"`, use `gateway.auth.password` (or `CLAWDBOT_GATEWAY_PASSWORD`).
+
+## Choosing an agent
+
+Target a specific Clawdbot agent by id:
+
+- `x-clawdbot-agent-id: <agentId>` (default: `main`)
+
+Advanced:
+- `x-clawdbot-session-key: <sessionKey>` to fully control session routing.
+
+## Session behavior
+
+By default the endpoint is **stateless per request** (a new session key is generated each call).
+
+If the request includes an OpenAI `user` string, the Gateway derives a stable session key from it, so repeated calls can share an agent session.
+
+## Streaming (SSE)
+
+Set `stream: true` to receive Server-Sent Events (SSE):
+
+- `Content-Type: text/event-stream`
+- Each event line is `data: <json>`
+- Stream ends with `data: [DONE]`
+
+## Examples
+
+Non-streaming:
+```bash
+curl -sS http://127.0.0.1:18789/v1/chat/completions \
+  -H 'Authorization: Bearer YOUR_TOKEN' \
+  -H 'Content-Type: application/json' \
+  -H 'x-clawdbot-agent-id: main' \
+  -d '{
+    "model": "clawdbot",
+    "messages": [{"role":"user","content":"hi"}]
+  }'
+```
+
+Streaming:
+```bash
+curl -N http://127.0.0.1:18789/v1/chat/completions \
+  -H 'Authorization: Bearer YOUR_TOKEN' \
+  -H 'Content-Type: application/json' \
+  -H 'x-clawdbot-agent-id: main' \
+  -d '{
+    "model": "clawdbot",
+    "stream": true,
+    "messages": [{"role":"user","content":"hi"}]
+  }'
+```
+