fix(deepagents): use langchain run.subagents instead of bespoke transformer (#598)

## Summary
- Remove deepagents' bespoke `createSubagentTransformer` (~460 lines)
and wire `createDeepAgent` to langchain's native `run.subagents`
projection from `createAgent` (langchain#37739).
- Slim `stream.ts` to type-only exports: re-export `SubagentRunStream`
from langchain and keep `DeepAgentRunStream` as a compile-time narrowing
over declared subagent specs.
- Update streaming tests for `sub.cause` (replacing `taskInput`), add
single-subagent `sub.messages` coverage, and document parallel-subagent
streaming behavior.
- Pin `langchain@1.4.6-dev-1781497519109` and `@langchain/langgraph` /
`@langchain/langgraph-checkpoint` to `d2312cf` dev builds (root
`pnpm.overrides`) for end-to-end verification until published releases
are available.
- Clarify `streamTransformers` docs: custom transformers surface on
`run.extensions`; built-in streams like `run.subagents` and
`run.toolCalls` are separate.

fixes #560