docs: expose webfetch companion guidance

[freelw]

Summary

• add optional webfetch-cli guidance to the browser-cli Skill and packaged Skill
• mention webfetch-cli capability checks in the packaged OpenAI agent prompt
• add doctor/prompt tests so the WebFetch exposure remains present without becoming a hard dependency

Validation

• uv run pytest
• uv run ruff check .
• git diff --check