docs: make the first Aiython demo work on a clean clone

[sunmodza]

Why

A fresh clone could not run the no-key --explain demo: the checked-in config required a gitignored credential file that was absent. This is a launch-time conversion blocker.

Changes

• Remove that env_file reference from the checked-in config. api_key_env remains; running aiython setup with a key can create a private file locally.
• Add a clean-checkout config regression test.
• Make the README explain the product in one line, show a no-key first try, and disclose provider cost/security before running it.
• Add assets/readme/social-preview.png (1280×640, 45 KB) for upload via GitHub Settings → Social preview after merge. Merely storing the file in the repo does not activate the preview.

Verification

• uv run --locked aiython --explain examples/recipes/03_loop.py succeeds without a credential and identifies two AI blocks.
• uv run --locked python -m unittest tests.test_config tests.test_cli_commands tests.test_ci_docs_only -q: 18 passed.
• git diff --check: clean.
• Full Windows suite: 587 run, 6 failures, 31 errors, 8 skipped. Most errors involve unsupported Windows symlinks/locked SQLite files or missing optional A2A dependency; not claiming the full suite is green. CI on this PR should be checked independently.