cli: rename langgraph.json to langsmith.json

Rename the default config filename from `langgraph.json` to
`langsmith.json` as part of the "LangGraph Platform" to "LangSmith
Deployment" rebranding.

Backward compatibility is preserved: the CLI falls back to
`langgraph.json` with a deprecation warning when `langsmith.json`
is not found. A new `_resolve_config` callback handles the
resolution logic for all `--config` click options.

Changes span libs/cli (core logic, examples, tests), libs/sdk-py
(docstrings), libs/langgraph (Makefile), and .github CI scripts.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

libs/cli/examples/langsmith.json

@@ -0,0 +1,17 @@
+{
+  "pip_config_file": "./pipconf.txt",
+  "dependencies": [
+    "langchain_community",
+    "langchain_anthropic",
+    "langchain_openai",
+    "wikipedia",
+    "scikit-learn",
+    "./graphs"
+  ],
+  "keep_pkg_tools": false, 
+  "graphs": {
+    "agent": "./graphs/agent.py:graph",
+    "storm": "./graphs/storm.py:graph"
+  },
+  "env": ".env"
+}