Refresh the README and add the rook mascot

[samyakLambda]

Makes the README look like a production public repo and adds the rook mascot.

What changed

• Header. Centred mascot, tagline, badges (npm version, Homebrew tap, test-scripts status, licence, platforms) and a quick-links row. The mascot is a <picture>: the black rook in GitHub's light theme, the cream rook in dark. Both are transparent SVGs under .github/assets/, made from the "Logo Cropped" artwork in the Rook Media drive folder with the background rectangle removed. The geometry is unchanged.
• Layout. The pre-alpha status and "point it at staging" are now GitHub callouts. The 18 scenario categories are a three-row table. The skill's alternative install paths are in a collapsible section. The long contents list is gone, replaced by the links row and GitHub's built-in outline.
• Install. The Homebrew section is down to the command and one paragraph. The old-tap migration steps now link to homebrew-rook, which already documents them in full.
• Samples. The README and samples/README.md now list all six samples. trip-weather-station and knowledge-vault were missing from both, and incident-scribe was missing from the README.
• Fix. samples/knowledge-vault told readers to cd sample/case-studies/knowledge-vault, a path that does not exist in this repository.

The product wording is unchanged apart from trimming. The #install, #for-ai-coding-agents-reading-this and #a-note-on-safety headings are unchanged because the skill, the installer README and the user guide link to them.

Checked

• Rendered both themes through GitHub's markdown API and headless Chrome. The mascot, badges, tables and collapsible section render correctly. Callouts render as GitHub alerts.
• Every relative link and in-page anchor in the changed files resolves.
• The branch merges cleanly with current main (63fe01f).

A companion PR gives LambdaTest/homebrew-rook the same treatment.

🤖 Generated with Claude Code