Docs: Fix broken LLM environment variable examples in README and USAGE

[MozzamShahid]

File / lines: README.md lines 83-95; docs/USAGE.md lines 127-135

Problem: Both guides show:

OPENAI_API_KEY=...
LLM_MODEL=gpt-5-mini

uv run gmaps-scraper ...

In POSIX shells, unexported variables set on separate lines are not inherited by the later uv run command, so the scraper sees an empty OPENAI_API_KEY and LLM_MODEL.

Suggested fix:

1. Use export OPENAI_API_KEY=... / export LLM_MODEL=..., or
2. Put the variables on the same line as the command, or
3. Switch the example to a .env file with --llm-env-file .env.

Make the same change in both README.md and docs/USAGE.md.

Difficulty: good first issue
Impact: Medium — high confusion for new users trying to enable LLM repair.