Add unit test support for plugins

[saeedvaziry]

What

Adds PHP unit-testing support for plugins. Plugin code compiles against host VitoDeploy classes (App\Plugins\*, App\SiteFeatures\Action, App\Models\*, the SSH facade) and the host's Tests\TestCase, so tests can't run standalone in this repo. This wires up a runner that stages each plugin into a checkout of vitodeploy/vito and runs the host's PHPUnit there — giving tests the real host classes and the auto-provisioned $this->server / $this->site.

Changes

• scripts/test.mjs — for each plugin with a tests/ dir: derives the host path from the manifest namespace, stages the plugin into app/Vito/Plugins/<Vendor>/<Name>/ and its tests into tests/Feature/Plugins/<Vendor>/<Name>/, runs the host's PHPUnit scoped to that dir, then cleans up (leaves zero residue). Exposed as npm test.
• .github/workflows/test.yml — required PR check (read-only token, same trust model as validate): checks out the host at 4.x, composer install, runs the changed plugins' tests. Failure blocks merge.
• Example tests for all three plugins (octane, reverb, tiny-file-manager) + a hello-world starter template.
• Docs — "Test your plugin" section in CONTRIBUTING.md; decision recorded in DESIGN.md.

Tests are opt-in per plugin (a plugin with no tests/ is reported as skipped, not failed) but required when present. tests/ is already excluded from the published artifact, so test files never ship.

Notes for reviewers

• The host uses PHPUnit, not Pest — pestphp/pest isn't a direct dep of vitodeploy/vito and there's no pest binary, so tests are class-based extending Tests\TestCase, matching the host's actual tests/Feature convention.
• A real plugin bug surfaced: Octane Enable::handle() renders view('ssh.services.webserver.nginx.vhost-blocks.laravel-octane'), which neither the plugin nor the host ships — a full enable throws View not found. The Octane example test is scoped to the parts the Action owns (validation, active() guard), with the vhost-view dependency documented in a comment. Worth a separate follow-up.
• Host ref is pinned to 4.x; add a matrix over ref to also gate an older line.

Verification

$ VITO_PATH=/path/to/vito node scripts/test.mjs
✓ laravel-octane-plugin
✓ laravel-reverb-plugin
✓ tiny-file-manager-plugin
3 plugin(s) tested, all passing.

validate and pack --dry-run still pass; pack output confirmed to exclude tests/.