Setting the LLM env vars now skips the matching CLI selection step and uses
the value, so OpenAI-compatible endpoints (opencode, LM Studio, etc.) and
unattended runs work without prompting. Unset vars are chosen interactively
as before.
TRADINGAGENTS_LLM_PROVIDER -> skips provider step (still verifies API key)
TRADINGAGENTS_LLM_BACKEND_URL -> custom endpoint (else provider default)
TRADINGAGENTS_DEEP_THINK_LLM / _QUICK_THINK_LLM -> skips model step
TRADINGAGENTS_OUTPUT_LANGUAGE -> skips language step
Builds on the existing TRADINGAGENTS_* config overrides (which already feed
DEFAULT_CONFIG); this wires the CLI to honor them instead of re-prompting.
