Godot plugin: release, list in the Godot Asset Library, and link from the README #284

Summary

Ship it: version the plugin, list it in the Godot Asset Library, and link it from this repo. This closes #116's "Done when".

Depends on #279, #280, #281 and #282 (and #283 green).

Suggested approach

• Semantic version in plugin.cfg, tagged vX.Y.Z in the plugin's repo, with a changelog.
• Asset Library submission:
  • category Tools
  • licence from Godot plugin: decide where it lives, its licence, and the minimum Godot version #276
  • icon, plus 2 or 3 screenshots (browse, details, CREDITS export)
  • a description that states where the data comes from and that licences are as recorded, to be checked before shipping
  • the minimum Godot version
• Make sure the download installs only addons/free_game_dev_assets/ (via .gitattributes export-ignore for repo-only files).
• In this repo: a README line and a section in docs/godot-budget-stack.md, plus a mention on the site's Godot pages.

Acceptance criteria

• The Asset Library listing is approved and installs into a fresh project at the minimum and latest Godot versions
• The README and the Godot guide link to it
• bigger bet: browse the catalog from inside Godot (then Unity / Unreal) with an editor plugin #116 is closed, with Unity and Unreal left as separate future ideas

_{Part of #116. Effort: S. audit-key: enhancement|godot-plugin|release}