[AI] Add knip to detect unused files, dependencies and exports (#8309)

Adds the knip tool with a monorepo-tuned config (knip.json) and a CI gate
in check.yml, plus the dead-code and dependency cleanup it surfaced. The
unused-export/type/duplicate rules are disabled so exporting an unused
symbol is allowed.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

.github/workflows/check.yml

@@ -55,6 +55,21 @@ jobs:
           download-translations: 'false'
       - name: Lint
         run: yarn lint
+  knip:
+    needs: setup
+    runs-on: depot-ubuntu-latest
+    steps:
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+        with:
+          persist-credentials: false
+      - name: Set up environment
+        uses: ./.github/actions/setup
+        with:
+          download-translations: 'false'
+      - name: Build plugins service
+        run: yarn build:plugins-service
+      - name: Check for unused files, dependencies and exports
+        run: yarn knip
   typecheck:
     needs: setup
     runs-on: depot-ubuntu-latest