[production] Build once, deploy to DoIT and Vault R2 test builds

[mrdavidgagnon]

Moves this repo's CI to the pattern already running on Aqualab (fielddaylab/wake#47, fielddaylab/wake#48): one Unity build, deployed independently to DoIT (as today) and to Vault's R2 test-build storage.

build   (VaultLearningGames/vault-publisher unity-build.yml@v1)
  ├─► doit     VPN + rsync → https://fielddaylab.wisc.edu/play/headlines/ci/<branch>/   (unchanged; delete this job when DoIT is retired)
  └─► preview  → https://builds.vaultlearninggames.org/fieldday/headlines/<branch>/
remove-preview (on branch delete)

What changes: only the workflow file. The build itself moves to the shared reusable workflow (same game-ci builder, Unity version read from ProjectSettings/ProjectVersion.txt); the DoIT upload becomes its own job using the same secrets and folder as before; a new job publishes the same build to the Vault CDN as game headlines of studio fieldday. Nothing in the game changes.

Headlines specifics: production is the only up-to-date branch here, so this is the only PR. develop, staging and main (the repo's default branch) are 11 to 18 commits behind production and still carry old workflows that GitHub now fails (actions/cache@v2, ubuntu-20.04); they get this workflow when production is merged or fast-forwarded into them. Merging rebuilds and redeploys the live game on DoIT (same code, new build). The test branch name does not start with production, so its test build is a development build; the build on production itself is a PRODUCTION build, as it is today.

What merging triggers: a push to production, so one Unity build (uses a Unity seat; the first build on a branch has no Library cache, so it is slower than usual), then

• DoIT: redeploys https://fielddaylab.wisc.edu/play/headlines/ci/production/ (same code, new build)
• Vault: publishes https://builds.vaultlearninggames.org/fieldday/headlines/production/

From then on every branch push does both, and deleting a branch removes its Vault build.

How to verify:

• This PR's own test build (branch vault-r2-publish, check below): https://builds.vaultlearninggames.org/fieldday/headlines/vault-r2-publish/ and https://fielddaylab.wisc.edu/play/headlines/ci/vault-r2-publish/
• After merging: the run on production in Actions shows build, doit and preview / publish green, and https://builds.vaultlearninggames.org/fieldday/headlines/production/ loads the game.

Merge with a merge commit or squash, one repo at a time (Unity seats). The first run creates a vault-staging deployment environment in this repo, which is where the published URL shows up.

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