chore: add changeset for 1.0.0

Author: nhuang-lc

.changeset/bright-spoons-sneeze.md

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+---
+"deepagents": major
+---
+
+Bring deepagentsjs up to date with latest 1.0.0 versions of LangChain and LangGraph. Add pluggable backends as well.
+
+DeepagentsJS now relies on middleware intead of built in tools.
+createDeepAgent's signature has been brought in line with createAgent's signature from LangChain 1.0.
+
+createDeepAgent now accepts a `backend` field in which users can specify custom backends for the deep agent filesystem.