Call for testers: v6.0.0-beta.1 (thin wrapper around game-ci/cli)

We just cut [`v6.0.0-beta.1`](https://github.com/game-ci/unity-builder/releases/tag/v6.0.0-beta.1) — a preview of the next major version of `unity-builder`, rewritten as a thin wrapper around [`game-ci/cli`](https://github.com/game-ci/cli) (see [#844](https://github.com/game-ci/unity-builder/pull/844) for the full technical writeup). It's tagged separately from `@v5`/`@v4` — nobody's existing workflow is affected by this.

Before we cut the real `v6`, we'd like people to actually try it against real projects. It's had unit tests and local checks, but not a live runner with a real build.

## How to try it

Point one workflow (or a copy of one) at the pre-release tag, and pin `cliVersion` explicitly so your run is deterministic (verified against `game-ci/cli` `v0.1.8` — cli's `main` is only a few commits ahead as of this writing and none of them touch the `build`/`activate` command surface, but pinning avoids depending on that staying true for the whole beta window):

```yaml
- uses: game-ci/unity-builder@v6.0.0-beta.1
with:
targetPlatform: WebGL
cliVersion: v0.1.8
```

Everything else — inputs, secrets, project layout — stays the same as `@v5`. If your build works the same, that's exactly the signal we're after. If it doesn't, that's even more useful to know now.

## Known gaps already called out (not surprises)

- `unityVersion` overrides are ignored — the CLI auto-detects from `ProjectSettings/ProjectVersion.txt` (you'll get a warning in the log if you set it explicitly).
- `providerStrategy` values other than `"local"` throw — same behavior as `@v5` without the orchestrator plugin installed, not new.

Full list in the [release notes](https://github.com/game-ci/unity-builder/releases/tag/v6.0.0-beta.1).

## What to report here

- Did your build succeed/fail, and how did that compare to `@v5`?
- Unity version, target platform(s), and runner OS you tested with.
- Anything that felt different, even something minor.

Thanks for helping get this solid before it ships as the default. 🙏

---

### Ready-to-post Discord announcement

If you're active in the [GameCI Discord](https://discord.gg/game-ci) and want to help spread the word, here's a copy-pasteable version for the announcement/help channel:

```
Hey folks! We just cut a pre-release of the next major `unity-builder` version — v6.0.0-beta.1. It's a rewrite as a thin wrapper around game-ci/cli instead of the action doing its own Docker orchestration. Tagged separately from @v5, so it won't touch anyone's existing workflows.

If you've got a spare few minutes, we'd love for you to try it against a real project before the real v6 ships:

- uses: game-ci/unity-builder@v6.0.0-beta.1
with:
targetPlatform: WebGL
cliVersion: v0.1.8

Everything else stays the same as @v5 - just swap the tag and add cliVersion so your run is deterministic. Known gaps: unityVersion overrides are ignored right now (auto-detected from ProjectSettings/ProjectVersion.txt instead), and providerStrategy other than "local" isn't supported yet (same as today without the orchestrator plugin).

Report back in the tracking issue: https://github.com/game-ci/unity-builder/issues/845

Thanks for helping kick the tires! 🙏
```

Start with the v6.0.0-beta.1 release notes and PR #844, then run a copied workflow using game-ci/unity-builder@v6.0.0-beta.1 with cliVersion v0.1.8. Test it against a real Unity project and compare the result with @v5. Done means reporting success or failure, Unity version, target platform, runner OS, and any differences here.

Written by the indexing model from the issue text.