Package keplergl-map skill as a Codex plugin

[Copilot]

Scaffolds the keplergl-map skill as a Codex plugin, making it installable from this repo and ready for marketplace publishing when the Plugin Directory opens.

Changes

• .codex-plugin/plugin.json — Plugin manifest with package identity, discovery metadata (keywords, category, defaultPrompt), and skills pointer to ./skills/
• skills/keplergl-map/ — Bundles the SKILL.md and all 7 layer-type reference docs (point, arc/line, geojson, h3, heatmap, hexbin, trip) from the skill branch
• assets/.gitkeep — Placeholder for future branding assets; composerIcon/logo fields are omitted from the manifest until real images exist

Plugin layout

.codex-plugin/
  plugin.json              # required entry point
skills/
  keplergl-map/
    SKILL.md
    skill-references/
      point-map.md
      geojson-polygon-map.md
      h3-hexagon-map.md
      arc-line-map.md
      heatmap.md
      hexbin-aggregation-map.md
      trip-animation-map.md
assets/
  .gitkeep

Before publishing

• Add icon.png / logo.png to assets/ and wire up composerIcon / logo in the manifest
• Self-serve publishing to the official Plugin Directory is not yet available upstream