fix(deepagents): propagate subagent lc_agent_name for delegated tasks (#566)

## Summary

Fixes #206.

This updates deepagents subagent delegation so tool executions can
reliably identify the active subagent via
`config.metadata.lc_agent_name` instead of inheriting the parent agent
name. It also adds focused regression tests that validate both compiled
subagents and standard subagent specs follow the same metadata behavior.

## Changes

### `libs/deepagents` subagent metadata propagation

- Updated `createTaskTool` subagent invocation config to explicitly set:
  - `metadata.lc_agent_name = subagent_type`
- existing `configurable.ls_agent_type = "subagent"` behavior remains
unchanged
- Added new regression coverage in `subagent.test.ts`:
  - compiled subagent (`runnable`) path
  - standard subagent spec (`systemPrompt/tools/model`) path
- Tests assert tool-time metadata receives the delegated subagent name
(`worker`), preventing parent-name leakage.

### `libs/deepagents` dependency range update

- Bumped `langsmith` peer dependency range in
`libs/deepagents/package.json` from `>=0.6.0 <1.0.0` to `^0.7.1`.
- Updated `pnpm-lock.yaml` accordingly.