chore: retire orchestrator-plugin CI checks superseded by cli-plugin arch

validate-orchestrator.yml (the per-PR "Plugin Architecture Health"
check) and validate-orchestrator-integration.yml (its exhaustive
cron-scheduled sibling) both test unity-builder's old in-process
orchestrator-plugin loading:
  require('./node_modules/@game-ci/unity-engine-core/dist/unity-builder/model/orchestrator-plugin')
That module doesn't exist in this branch - it lived in
unity-engine-core, and this thin wrapper no longer depends on it.

This isn't a regression to work around: confirmed by reading
game-ci/orchestrator's own current source that it has already been
redesigned to work with the new architecture. It now ships a
`cli-plugin` export (src/cli-plugin/index.ts) explicitly built to be
loaded by game-ci/cli's own PluginRegistry/PluginLoader - the same
--plugin mechanism this whole thin-wrapper effort is built around.
Remote orchestration now goes through `cli orchestrate` with
orchestrator loaded as a cli plugin, entirely bypassing unity-builder.

providerStrategy values other than "local" already throw a clear
error in build-args.ts pointing at this - action.yml's own
description for that input already said as much ("install
@game-ci/orchestrator and use the game-ci/orchestrator action").

Also removed the now-orphaned src/types/game-ci-orchestrator.d.ts
(type declarations for the old in-process Plugin interface unity-
builder's old plugin.ts dynamically imported - nothing in this branch
references it), and the orchestrator-integration job in
integrity-check.yml that called the now-deleted workflow.

.github/workflows/integrity-check.yml

@@ -53,10 +53,3 @@ jobs:
       - run: bash <(curl -s https://codecov.io/bash)
       - run: yarn build || { echo "build command should always succeed" ; exit 61; }
   #      - run: yarn build --quiet && git diff --quiet dist || { echo "dist should be auto generated" ; git diff dist ; exit 62; }
-
-  orchestrator-integration:
-    name: Orchestrator Integration
-    if: >-
-      github.event_name == 'push' || contains(github.event.pull_request.labels.*.name, 'run-integration')
-    uses: ./.github/workflows/validate-orchestrator-integration.yml
-    secrets: inherit