docs: align skill readme with skills template

[FrancescoSaverioZuppichini]

Summary

• Refactor skills/just-scrape/SKILL.md to mirror the raw Browserbase skill template structure
• Make the root README repo-focused: scope, stack, repository layout, dev commands, small usage examples, and a link to the skill file
• Remove old API/migration wording and keep examples aligned with the current CLI command args

Verification

• git diff --check -- README.md skills/just-scrape/SKILL.md
• Checked command examples against src/commands/* and CLI help where the local SDK install allowed it
• Confirmed branch diff against main contains only README.md and skills/just-scrape/SKILL.md

Notes

• Did not commit scripts or the existing unstaged .gitignore/docs changes