ci(shorebird): remove shard_runner package; now lives in _build_engine (#145)

The shard_runner package moved to shorebirdtech/_build_engine in
shorebirdtech/_build_engine#209. Runner code lives next to the
workflows that invoke it; runner changes no longer need a Flutter PR
+ engine bump round-trip to land + test.

What stays in this repo (still read by the runner via --config-dir):
- shorebird/ci/shards/*.json
- shorebird/ci/compose.json
- shorebird/ci/artifacts_manifest.template.yaml

Also drops the 🧩 Shard Runner Tests job from shorebird_ci.yml and
the matching entry from the `ci` aggregator's needs list — those tests
now run in _build_engine's CI.

Depends on shorebirdtech/_build_engine#209 landing first; otherwise
in-flight engine builds against pre-bump SHAs would be unaffected
(workflow points at _build_engine's runner) but anyone iterating
locally on the runner would be confused by the deleted dir.

Author: eseidel

.github/workflows/shorebird_ci.yml

@@ -38,34 +38,6 @@ jobs:
         run: ../../bin/flutter test test/general.shard
         working-directory: packages/flutter_tools
 
-  shard-runner-tests:
-    runs-on: ubuntu-latest
-
-    name: 🧩 Shard Runner Tests
-
-    defaults:
-      run:
-        working-directory: shorebird/ci/shard_runner
-
-    steps:
-      - name: 📚 Git Checkout
-        uses: actions/checkout@v4
-
-      - name: 🎯 Setup Dart
-        uses: dart-lang/setup-dart@v1
-
-      - name: 📦 Install Dependencies
-        run: dart pub get
-
-      - name: 🔍 Check Formatting
-        run: dart format --output=none --set-exit-if-changed .
-
-      - name: 🔎 Analyze
-        run: dart analyze --fatal-infos
-
-      - name: 🧪 Run Tests
-        run: dart test
-
   shorebird-android-tests:
     # Android tests run on Ubuntu (faster runners, no macOS needed)
     runs-on: ubuntu-latest
@@ -148,7 +120,6 @@ jobs:
     if: always()
     needs:
       - flutter-tools-tests
-      - shard-runner-tests
       - shorebird-android-tests
       - shorebird-ios-tests
     runs-on: ubuntu-latest