feat: add HTTP health endpoint on gateway port

Binds a lightweight asyncio HTTP server on the configured gateway
port (default 18790) alongside the existing agent and channel tasks.

Endpoints:
  GET /       -> "nanobot" (plain text, for service discovery)
  GET /health -> JSON with service, version, status, uptime, channels

Zero new dependencies — uses asyncio.start_server.

nanobot/cli/commands.py

@@ -674,6 +674,67 @@ def gateway(
 
     console.print(f"[green]✓[/green] Heartbeat: every {hb_cfg.interval_s}s")
 
+    async def _health_server(host: str, health_port: int):
+        """Lightweight HTTP health endpoint on the gateway port."""
+        import json as _json
+        import time
+
+        start_time = time.monotonic()
+
+        async def handle(reader, writer):
+            try:
+                data = await asyncio.wait_for(reader.read(4096), timeout=5)
+            except (asyncio.TimeoutError, ConnectionError):
+                writer.close()
+                return
+
+            request_line = data.split(b"\r\n", 1)[0].decode("utf-8", errors="replace")
+            method, path = "", ""
+            parts = request_line.split(" ")
+            if len(parts) >= 2:
+                method, path = parts[0], parts[1]
+
+            if method == "GET" and path == "/health":
+                uptime_s = int(time.monotonic() - start_time)
+                body = _json.dumps({
+                    "service": "nanobot",
+                    "version": __version__,
+                    "status": "running",
+                    "uptime_seconds": uptime_s,
+                    "channels": channels.enabled_channels,
+                })
+                resp = (
+                    f"HTTP/1.0 200 OK\r\n"
+                    f"Content-Type: application/json\r\n"
+                    f"Content-Length: {len(body)}\r\n"
+                    f"\r\n{body}"
+                )
+            elif method == "GET" and path == "/":
+                body = "nanobot"
+                resp = (
+                    f"HTTP/1.0 200 OK\r\n"
+                    f"Content-Type: text/plain\r\n"
+                    f"Content-Length: {len(body)}\r\n"
+                    f"\r\n{body}"
+                )
+            else:
+                body = "Not Found"
+                resp = (
+                    f"HTTP/1.0 404 Not Found\r\n"
+                    f"Content-Type: text/plain\r\n"
+                    f"Content-Length: {len(body)}\r\n"
+                    f"\r\n{body}"
+                )
+
+            writer.write(resp.encode())
+            await writer.drain()
+            writer.close()
+
+        server = await asyncio.start_server(handle, host, health_port)
+        console.print(f"[green]✓[/green] Health endpoint: http://{host}:{health_port}/health")
+        async with server:
+            await server.serve_forever()
+
     async def run():
         try:
             await cron.start()
@@ -681,6 +742,7 @@ def gateway(
             await asyncio.gather(
                 agent.run(),
                 channels.start_all(),
+                _health_server(config.gateway.host, port),
             )
         except KeyboardInterrupt:
             console.print("\nShutting down...")