docs(onboarding): add provider env starters

[typelicious]

What changed

• add provider .env starter files for cloud openai-compat, local-worker, and image-provider setups
• link the env starters from onboarding, integrations, and README helper docs
• keep the provider templates and env examples paired for copy/paste onboarding

Why

• make provider onboarding complete without having to invent matching env keys by hand
• reduce friction between provider config snippets and real local setup

How verified

• ./.venv-check-313/bin/ruff check README.md docs/INTEGRATIONS.md docs/ONBOARDING.md CHANGELOG.md
• git diff --check
• python3 sanity check for new docs/examples env files