feat(autobrowse): support OpenAI-compatible providers

[skippyom]

Summary

• keep Anthropic as the default provider for scripts/evaluate.mjs
• add --provider openai / AUTOBROWSE_PROVIDER=openai for OpenAI-compatible Chat Completions providers
• support OpenAI, OpenRouter, LiteLLM, and similar gateways via OPENAI_BASE_URL
• normalize OpenAI tool calls back into the harness' existing Anthropic-style trace/message loop
• document provider configuration in README, REFERENCE, SKILL, and .env.example

Test Plan

• npm install
• node --check scripts/evaluate.mjs
• node scripts/evaluate.mjs --help with assertions for provider/OpenAI help text
• Anthropic missing-key error path still reports ANTHROPIC_API_KEY
• OpenAI missing-key error path reports OPENAI_API_KEY
• OpenAI-compatible smoke test against a local mock Chat Completions server that returns a tool call for browse status, then a final JSON answer
• git diff --check

Notes

• No new runtime dependency is required for OpenAI-compatible mode; it uses Node 18+ global fetch.
• OpenRouter-specific optional headers are supported through OPENAI_SITE_URL and OPENAI_APP_NAME.