feat(paper): bundle plugin-grounds-platform v0.1.0

[hbrombeer]

Summary

Adds the in-pod platform-context plugin to every `paper` image build. At runtime the plugin reads the four `GROUNDS_*` env vars that grounds-forge 1.9.0+ sets on Minecraft deploys and:

• replaces the default MOTD with `\n§8via Grounds`,
• polls `GET /v1/projects//whitelist` every 30s with the workload-scoped token and reconciles Paper's `whitelist.json`.

If the env vars are missing (older forge, non-MC pod) the plugin logs one WARN and stays inert — user gameplay isn't affected.

Distribution choice

`plugin-grounds-platform` ships as a public GitHub Release asset, not GitHub Packages Maven, so this PR fetches it with a direct `curl` rather than through `download-plugins.sh`. That avoids requiring a `github_token` secret for the artifact and keeps the plugin reachable from forks of this repo. Once the plugin starts publishing to GHCR Maven the curl step can collapse into the `PLUGINS` list above.

The version is exposed as `GROUNDS_PLATFORM_PLUGIN_VERSION` so a future bump is a one-line change.

Test plan

• After CI builds the image, push from `grounds-push/sample` (`grounds push push --target=dev`) and verify in the Paper console:
  • `Started platform plugin successfully (projectId=..., forgeUrl=..., pollSeconds=30)`
  • The MOTD shown to a connecting client is the project name with `via Grounds` underneath.
  • `POST /v1/projects//whitelist {minecraftName: "Notch"}` then within 30s: `Whitelist add applied (uuid=..., username=Notch)` in the console; `Notch` can join.

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