fix: stop mutating OPENAI env during model setup

[fmueller]

Summary

• remove OpenAISettings.configure_environment() so model setup no longer mutates process-wide OPENAI_* variables
• update idea/brief/post-edit agent creation paths to rely on explicit provider settings only
• replace the environment-mutation unit test with a regression test asserting generate_brief leaves OPENAI_* untouched

Validation

• uv run ruff check
• uv run mypy
• uv run pytest