moon CI on GitHub Actions

moon runs every task because the toolchain and cache are cold.

Why it happens

.moon/cache and the proto toolchain directory are discarded between runs.

What to change

1. Cache .moon/cache and ~/.proto keyed on the lockfiles
2. Use moon ci to run only affected tasks with correct base detection
3. Fetch full git history so affected detection works

moon on runnerhut

jobs:
  ci:
    runs-on: runnerhut-8vcpu-ubuntu-2404
    steps:
      - uses: actions/checkout@v4
        with: { fetch-depth: 0 }
      - run: npx moon ci

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.