fix: add plugin logo using the official planetscale logomark

[heyglassy]

Summary

The plugin has no logo — .cursor-plugin/plugin.json never declared one and the repo contains no image, so Cursor has nothing to render for it. Adds the logo field and the asset:

  "description": "An authenticated hosted MCP server that accesses ...",
+ "logo": "assets/logo.svg",
  "mcpServers": "./.mcp.json",

assets/logo.svg is the official logomark from https://planetscale.com/brand, path data copied verbatim rather than redrawn, on a brand-black #1a1a1a square:

• White mark (#fafafa) on filled #1a1a1a — Cursor's logo takes a single path, with no light/dark variants, so a transparent mark would disappear on one of the two themes.
• 512×512 with the 454×454 mark scaled to 360 and centered. The brand downloads ship with no clear space, so the ~15% padding is added here; without it the corners of the diagonal get clipped wherever the logo is rendered rounded or circular.

The path is relative, which Cursor resolves to a raw.githubusercontent.com URL — so this needs no change to the release archive, and it's the form marketplace submission expects.

Stacks conceptually on top of #12 but touches no files in common with it.

Link to Devin session: https://app.devin.ai/sessions/3f256659316c425facc3ef65f3eb5c50
Open in Devin Desktop: https://app.devin.ai/desktop/session/3f256659316c425facc3ef65f3eb5c50?variant=devin
Requested by: @heyglassy

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Devin Review found 1 potential issue.