fix(deepagents): declare LangChain runtime packages as peer dependencies (#613)

Updates to `@langchain/core` can cause downstream friction with
TypeScript:

```
Type 'ChatOpenAI<ChatOpenAICallOptions>' is not assignable to type 'string | BaseLanguageModel<any, BaseLanguageModelCallOptions> | undefined'.
  Type 'ChatOpenAI<ChatOpenAICallOptions>' is not assignable to type 'BaseLanguageModel<any, BaseLanguageModelCallOptions>'.
    Types of property 'getGraph' are incompatible.
      Type '(_?: import("/Users/christian.bromann/Sites/LangChain/deployment-cookbook/js-langsmith/node_modules/.pnpm/@langchain+core@1.2.1_openai@6.44.0_ws@8.21.0_zod@4.4.3__ws@8.21.0/node_modules/@langchain/core/dist/runnables/types").RunnableConfig<Record<string, any>> | undefined) => import("/Users/christian.bromann/Sites/L...' is not assignable to type '(_?: import("/Users/christian.bromann/Sites/LangChain/deployment-cookbook/js-langsmith/node_modules/.pnpm/@langchain+core@1.2.0_openai@6.44.0_ws@8.21.0_zod@4.4.3__ws@8.21.0/node_modules/@langchain/core/dist/runnables/types").RunnableConfig<Record<string, any>> | undefined) => import("/Users/christian.bromann/Sites/L...'.
        Types of parameters '_' and '_' are incompatible.
          Type 'import("/Users/christian.bromann/Sites/LangChain/deployment-cookbook/js-langsmith/node_modules/.pnpm/@langchain+core@1.2.0_openai@6.44.0_ws@8.21.0_zod@4.4.3__ws@8.21.0/node_modules/@langchain/core/dist/runnables/types").RunnableConfig<Record<string, any>> | undefined' is not assignable to type 'import("/Users/christian.bromann/Sites/LangChain/deployment-cookbook/js-langsmith/node_modules/.pnpm/@langchain+core@1.2.1_openai@6.44.0_ws@8.21.0_zod@4.4.3__ws@8.21.0/node_modules/@langchain/core/dist/runnables/types").RunnableConfig<Record<string, any>> | undefined'.
            Type 'import("/Users/christian.bromann/Sites/LangChain/deployment-cookbook/js-langsmith/node_modules/.pnpm/@langchain+core@1.2.0_openai@6.44.0_ws@8.21.0_zod@4.4.3__ws@8.21.0/node_modules/@langchain/core/dist/runnables/types").RunnableConfig<Record<string, any>>' is not assignable to type 'import("/Users/christian.bromann/Sites/LangChain/deployment-cookbook/js-langsmith/node_modules/.pnpm/@langchain+core@1.2.1_openai@6.44.0_ws@8.21.0_zod@4.4.3__ws@8.21.0/node_modules/@langchain/core/dist/runnables/types").RunnableConfig<Record<string, any>>'.
              Types of property 'callbacks' are incompatible.
                Type 'import("/Users/christian.bromann/Sites/LangChain/deployment-cookbook/js-langsmith/node_modules/.pnpm/@langchain+core@1.2.0_openai@6.44.0_ws@8.21.0_zod@4.4.3__ws@8.21.0/node_modules/@langchain/core/dist/callbacks/manager").Callbacks | undefined' is not assignable to type 'import("/Users/christian.bromann/Sites/LangChain/deployment-cookbook/js-langsmith/node_modules/.pnpm/@langchain+core@1.2.1_openai@6.44.0_ws@8.21.0_zod@4.4.3__ws@8.21.0/node_modules/@langchain/core/dist/callbacks/manager").Callbacks | undefined'.
                  Type 'CallbackManager' is not assignable to type 'Callbacks | undefined'.
                    Type 'import("/Users/christian.bromann/Sites/LangChain/deployment-cookbook/js-langsmith/node_modules/.pnpm/@langchain+core@1.2.0_openai@6.44.0_ws@8.21.0_zod@4.4.3__ws@8.21.0/node_modules/@langchain/core/dist/callbacks/manager").CallbackManager' is not assignable to type 'import("/Users/christian.bromann/Sites/LangChain/deployment-cookbook/js-langsmith/node_modules/.pnpm/@langchain+core@1.2.1_openai@6.44.0_ws@8.21.0_zod@4.4.3__ws@8.21.0/node_modules/@langchain/core/dist/callbacks/manager").CallbackManager'.
                      The types returned by 'handleLLMStart(...)' are incompatible between these types.
                        Type 'Promise<import("/Users/christian.bromann/Sites/LangChain/deployment-cookbook/js-langsmith/node_modules/.pnpm/@langchain+core@1.2.0_openai@6.44.0_ws@8.21.0_zod@4.4.3__ws@8.21.0/node_modules/@langchain/core/dist/callbacks/manager").CallbackManagerForLLMRun[]>' is not assignable to type 'Promise<import("/Users/christian.bromann/Sites/LangChain/deployment-cookbook/js-langsmith/node_modules/.pnpm/@langchain+core@1.2.1_openai@6.44.0_ws@8.21.0_zod@4.4.3__ws@8.21.0/node_modules/@langchain/core/dist/callbacks/manager").CallbackManagerForLLMRun[]>'.
                          Type 'import("/Users/christian.bromann/Sites/LangChain/deployment-cookbook/js-langsmith/node_modules/.pnpm/@langchain+core@1.2.0_openai@6.44.0_ws@8.21.0_zod@4.4.3__ws@8.21.0/node_modules/@langchain/core/dist/callbacks/manager").CallbackManagerForLLMRun[]' is not assignable to type 'import("/Users/christian.bromann/Sites/LangChain/deployment-cookbook/js-langsmith/node_modules/.pnpm/@langchain+core@1.2.1_openai@6.44.0_ws@8.21.0_zod@4.4.3__ws@8.21.0/node_modules/@langchain/core/dist/callbacks/manager").CallbackManagerForLLMRun[]'.
                            Type 'import("/Users/christian.bromann/Sites/LangChain/deployment-cookbook/js-langsmith/node_modules/.pnpm/@langchain+core@1.2.0_openai@6.44.0_ws@8.21.0_zod@4.4.3__ws@8.21.0/node_modules/@langchain/core/dist/callbacks/manager").CallbackManagerForLLMRun' is not assignable to type 'import("/Users/christian.bromann/Sites/LangChain/deployment-cookbook/js-langsmith/node_modules/.pnpm/@langchain+core@1.2.1_openai@6.44.0_ws@8.21.0_zod@4.4.3__ws@8.21.0/node_modules/@langchain/core/dist/callbacks/manager").CallbackManagerForLLMRun'.
                              Property 'inheritableHandlers' is protected but type 'BaseRunManager' is not a class derived from 'BaseRunManager'.ts(2322)
```

to avoid this we should move the LangChain dependency to become peer
deps.

- Moved `@langchain/core`, `@langchain/langgraph`,
`@langchain/langgraph-sdk`, and `langchain` from `dependencies` to
`peerDependencies` in `libs/deepagents`, keeping dev copies for isolated
build/test.
- Lets consumers bump `@langchain/core` without requiring a new
`deepagents` release; a release is only needed to widen the peer range
across a major.
- Note on install contract: npm 7+ and pnpm 8+ auto-install peers; Yarn
only warns, so Yarn users must add the four packages explicitly (worth a
README follow-up).

---------

Signed-off-by: Christian Bromann <git@bromann.dev>
Co-authored-by: open-swe[bot] <215916821+open-swe[bot]@users.noreply.github.com>