Support configurable LLM providers

[caydyan]

/claim #10

Summary

• Add LLM_PROVIDER support for selecting azure_openai or openai from .env.
• Keep Azure OpenAI as the default provider for backward compatibility.
• Add OpenAI-compatible client setup using OPENAI_API_KEY and optional OPENAI_BASE_URL.
• Make environment validation provider-aware and reject unsupported providers early.
• Document the provider settings in .env.example and README.md.

Tests

python3 -m unittest tests/test_llm_provider_helpers.py
python3 -m compileall helpers main.py tests/test_llm_provider_helpers.py
git diff --check

Repository note

Algora links this bounty to daytonaio/devcontainer-generator#10, but that GitHub repo path currently returns 404/disabled issues. This PR targets nibzard/devcontainer-generator, matching the existing claim PR pattern in this repo.