Add langgraph.json configuration file

Fixes virattt#99

Add `langgraph.json` configuration file to enable running the application with the `langgraph` CLI.

* Add `langgraph.json` to the root directory with necessary configuration for `langgraph` CLI.
* Include settings for dependencies, graphs, environment variables, store index, Python and Node versions, pip config file, and additional Dockerfile lines.

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langgraph.json

@@ -0,0 +1,20 @@
+{
+  "dependencies": ["."],
+  "graphs": {
+    "hedge_fund": "./src/main.py:app"
+  },
+  "env": ".env",
+  "store": {
+    "index": {
+      "embed": "openai:text-embedding-3-small",
+      "dims": 1536,
+      "fields": ["$"]
+    }
+  },
+  "python_version": "3.11",
+  "node_version": "20",
+  "pip_config_file": "./pip.conf",
+  "dockerfile_lines": [
+    "RUN apt-get update && apt-get install -y libgomp1"
+  ]
+}