chore(plugin): align GitHub and Tasks built-in plugin icons

[smsunarto]

im sorry i have mild ocd

Summary

• remove the custom GitHub and Tasks branding logos
• use each plugin's existing named host icon in both the sidebar and Plugins settings
• remove the unused SVG assets from package contents

Why

The sidebar rendered these plugins through the host icon registry, while Plugins settings preferred their custom logo assets. This gave the same plugin different color, size, and stroke treatment across surfaces.

Validation

• pnpm exec prettier --check plugins/github/package.json plugins/tasks/package.json
• pnpm --filter bb-plugin-github typecheck
• pnpm --filter bb-plugin-tasks typecheck
• pnpm --filter bb-plugin-github test (6 passed)
• targeted app icon and plugin-detail tests (34 passed)
• pnpm --filter bb-plugin-github build
• pnpm --filter @bb/plugin-sdk build && pnpm --filter bb-plugin-tasks build

Visual comparison

Captured from the real Plugins settings page at the same viewport in development builds before and after this change.

[SawyerHood]

ty for this!