feat(brand): integrate new OCR logo/cover assets (READMEs + dashboard)

[spencermarx]

New brand assets in the READMEs + dashboard

Integrates the new OCR logo/cover set, replacing the old open-code-review-logo.png.

Changes

• Root README hero → the full-bleed OCR-Github-Cover.png banner (was a 400px logo).
• Package READMEs (cli, dashboard, agents) → a consistent, centered, theme-aware wordmark header via <picture> + prefers-color-scheme (dark ink on light, light ink on dark). Uses absolute raw.githubusercontent URLs so the images render on npm too (cli + agents are published — relative paths 404 there).
• Dashboard sidebar → the generic FileSearch glyph in the top-left brand is now the OCR monogram, inlined as a single OcrLogoIcon component with fill="currentColor". It follows the wordmark's color exactly and themes light/dark with zero JS and no extra asset request (geometry verbatim from OCR-Logo-Icon-*.svg).

Design notes

• One source for the mark (currentColor), not two theme variants — tonal harmony with the wordmark, and the brand color can't drift between themes.
• The cover keeps its white background; a dedicated dark-mode cover (via <picture>) is a possible future polish.
• Favicons in index.html are already theme-aware — left as-is; regenerating them from the new mark is a noted follow-up.

Verified

• Dashboard typecheck + build + unit tests green.
• Sidebar monogram confirmed rendering in both light and dark (screenshots below — black mark on light, white on dark, sized to the wordmark).