feat(ci): add initial eval suites

[Hunter Lovell (hntrl)]

Summary

Add a LangSmith-backed eval framework for deepagents using the langsmith/vitest integration. This provides a reusable harness (runAgent, custom matchers, AgentTrajectory types) and an initial suite of evals covering file operations, subagent delegation, and system prompt customization.

Port of https://github.com/langchain-ai/deepagents/pull/1400/changes

Changes

New: Eval harness (libs/deepagents/src/evals/index.ts)

• runAgent() helper that invokes a deepagent with a user query + optional pre-seeded files and returns a structured AgentTrajectory
• Custom vitest matchers: toHaveAgentSteps, toHaveToolCallRequests, toHaveToolCallInStep, toHaveFinalTextContaining
• All matchers log feedback to LangSmith automatically for experiment tracking
• Shared default agent instance using anthropic:claude-sonnet-4-5-20250929

New: Eval suites

• File operations (11 tests) — read, write, parallel read/write, edit/replace, ls directory, grep, glob, derived output, and avoiding unnecessary tool calls
• Subagents (2 tests) — delegating to a named custom subagent and to the general-purpose subagent
• System prompt (1 test) — verifying custom system prompt behavior

Config changes

• vitest.config.ts — new eval mode with 120s timeout, langsmith/vitest/reporter, and *.eval.test.ts include pattern; default mode now excludes *.eval.test.ts
• package.json — new test:eval script; added @langchain/anthropic and langsmith dev dependencies
• eslint.config.ts — disabled @typescript-eslint/no-empty-object-type globally and for **/evals/** files

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