Godot CI on GitHub Actions

Export templates download on every run and imports rerun each build.

Why it happens

The Godot editor cache and export templates are not persisted.

What to change

1. Cache the export templates directory keyed on the Godot version
2. Cache the .godot import directory so assets are not reimported
3. Export platforms as parallel matrix legs

Godot on runnerhut

jobs:
  export:
    runs-on: runnerhut-8vcpu-ubuntu-2404
    steps:
      - uses: actions/checkout@v4
      - run: godot --headless --export-release "Linux/X11" build/game

The only runnerhut-specific line is `runs-on: runnerhut-8vcpu-ubuntu-2404`. Everything else is standard GitHub Actions — the same actions, the same secrets and the same permissions model you use today.