Add cache-tax brand cards, icons, and README banner

[karanb192]

Summary

• site/brand/: five PNGs rendered from hand-written SVG by build.mjs (sharp): social card 1280x640, X card 1200x675, icon 512 in ice blue and a warm amber variant for the Mod's keep-warm state, favicon 32. SVG sources beside them, and a short README saying which file is for what. Served by Pages under /brand/ on merge, since the deploy workflow triggers on site/**.
• plugins/cache-tax/README.md: the social card as a banner under the title, relative path into site/brand/.
• package.json declares sharp ^0.35.4 under devDependencies, with a package-lock.json, and a build:brand script, so npm install && npm run build:brand reproduces the PNGs from any directory in a fresh clone. build.mjs writes its outputs next to itself, not into the working directory. build.mjs trims line-end whitespace before writing each SVG, so the sources pass git diff --check.
• Palette and type are the site's own variables (void, panel, text, block red, amber) plus an ice blue set for the cube. No emoji in the SVG, so the render does not depend on an emoji font.

Test plan

• git diff --check main clean; regenerated PNGs byte-identical to the previous commit; npm test 1,928 passing
• Each PNG opened and checked at full size: tagline fits on two lines, chips inside the frame, footer inside the frame on both card sizes, cube legible at 32 px
• After merge, https://hooks.karanbansal.in/brand/cache-tax-social-1280x640.png resolves and the plugin README shows the banner

[karanb192]

Two small fixes before this lands:

1. site/brand/build.mjs imports sharp, but package.json does not declare it. In a fresh clone, node site/brand/build.mjs fails with ERR_MODULE_NOT_FOUND. Please add it as a development dependency so the checked-in PNGs can be reproduced.

2. git diff --check main...HEAD reports trailing whitespace in all five generated SVGs. Please regenerate or clean the sources.

The assets themselves look good, the README banner path is correct, and npm test passes (1,928 tests).

Small metadata nit: the title is 74 characters. Add cache-tax brand cards, icons, and README banner is tighter.

[karanb192]

One remaining reproducibility issue. The PR body says npm install && node site/brand/build.mjs reproduces the assets from a fresh clone. Running that command from the repository root writes the ten SVG and PNG files into the repository root, not site/brand/, because build.mjs uses CWD-relative output paths.

Please resolve outputs relative to build.mjs itself, then add a root-level command such as npm run build:brand. That gives the repo one safe, documented regeneration path.