Make LLM backend any OpenAI-compatible endpoint

[ethenotethan]

Summary

• Drops the hardcoded OpenRouter base URL and Claude Agent SDK framing in favor of generic OpenAI Chat Completions config, so Flashlight works against OpenAI, OpenRouter, vLLM, LM Studio, Ollama, Together, Groq, etc.
• New env vars: OPENAI_API_KEY (required), OPENAI_BASE_URL (default https://api.openai.com/v1), OPENAI_MODEL (default gpt-4o-mini). No OPENROUTER_* backwards compat.
• README no longer references Claude Agent SDK / ANTHROPIC_API_KEY; adds an LLM configuration section with an env-var table and per-provider example configs.

Changes

• agent/burr_app.py — parametrized base URL via get_base_url(), renamed client helper call_openrouter → _chat_completion (avoids collision with the existing Burr call_llm action), OpenRouter-specific HTTP-Referer / X-Title attribution headers now only emitted when the base URL actually contains openrouter.ai.
• agent/cli.py, agent/agent.py — API-key checks swapped to OPENAI_API_KEY with provider-agnostic error messages.
• .env.example — rewritten with OPENAI_* vars and example configs for OpenAI / OpenRouter / local vLLM / LM Studio / Ollama.
• README.md — intro + Installation + Requirements + Contributing sections de-Claude'd; de-emoji'd verbose-logging bullets.

Compatibility constraint

"OpenAI-compatible" here specifically means the server returns structured tool_calls on the response message (not JSON emitted as text in content). Flashlight's ReAct loop relies on response["tool_calls"] in _chat_completion; an endpoint that only emits tool-call intent as plain text will stall the loop after one turn. Works well with OpenAI, OpenRouter (Claude/GPT/Gemini), vLLM, LM Studio, mlx-omni-server, and other servers that implement the OpenAI tools schema.

Test plan

• Syntax check: python -m compileall agent/
• Smoke test config resolution:

  OPENAI_API_KEY=dummy python -c "from agent.burr_app import get_api_key, get_base_url, DEFAULT_MODEL; print(get_api_key(), get_base_url(), DEFAULT_MODEL)"
  

• Missing-key path raises with new message: python -c "from agent.burr_app import get_api_key; get_api_key()"
• End-to-end with a real endpoint (OpenAI or OpenRouter) not run in this PR — reviewer-assisted.