Add environment variables for LLM authentication:
- ANTHROPIC_API_KEY and CLAUDE_SESSION_TOKEN for Claude Code tests
- OPENAI_API_KEY for Codex and OpenCode tests
Add preflight step that validates credentials are present before running
tests. The step exits with code 1 and clear error messaging if any
required secrets are missing, preventing long hangs from auth failures.
Follows STEER guidance: does NOT set CLAUDE_CONFIG_DIR, CODEX_HOME, or
CODEX_SESSION_DIR - relies on default auth paths.
