fix(deepagents): remove unconditional @langchain/anthropic import (#451)

## Summary

Fixes #443

- Removed static `import { ChatAnthropic } from "@langchain/anthropic"`
at module level
- Changed default `model` parameter from `new
ChatAnthropic("claude-sonnet-4-6")` to the string `"claude-sonnet-4-6"`

## Problem

`deepagents@1.9.0` unconditionally imported `@langchain/anthropic` at
the top of `agent.ts`. Bundlers follow static imports regardless of
which model the user actually uses, causing builds to fail for users who
only use other providers (e.g. `ChatOpenAI`) and don't have
`@langchain/anthropic` installed.

## Solution

`createAgent` from `langchain` natively accepts a model name string
(`string | AgentLanguageModelLike`), so no `ChatAnthropic` instantiation
is needed at the library level. With a string default, bundlers have no
static reference to `@langchain/anthropic` and can tree-shake it
entirely.

Users who want Anthropic models can either:
- Pass `new ChatAnthropic(...)` explicitly (requires
`@langchain/anthropic` in their own `package.json`)
- Rely on the `"claude-sonnet-4-6"` string default (also requires
`@langchain/anthropic` at runtime, but no longer breaks builds for
non-Anthropic users)

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Co-authored-by: Hunter Lovell <40191806+hntrl@users.noreply.github.com>