feat: add DeepSeek agent runner

[sanchitmehtagit]

Adds a new deepseek agent runner that drives DeepSeek models via the Codex SDK against an OpenAI-compatible Responses API endpoint. Routes through the configured proxy (DEEPSEEK_PROXY_BASE_URL env var, falling back to LLM_PROXY_BASE_URL) using the same LLM_API_KEY as all other runners.

• packages/evals/src/runners/deepseek/ - new runner (agent.ts + runner.ts)
• evals-core: add 'deepseek' to KNOWN_AGENT_TYPES, AGENT_CONTEXT_FILENAMES, DEEPSEEK_MODELS prefix, isDeepSeekModel() helper, cost table entries
• evals/cli: register DeepSeekRunner in run.ts and sandbox-runner.ts
• apps/auth0-evals/eval.config.js: add deepseek agent proxy config

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