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>
