ci(frontend): lint changed lines via eslint-plugin-diff

The previous job linted whole changed files, so touching a file with the
frontend's pre-existing eslint backlog failed the PR on errors it never
introduced — pushing contributors toward project-wide `lint:fix` (see #7733).

eslint-plugin-diff's `plugin:diff/ci` config filters lint messages to lines
changed vs the PR base. It is a no-op locally and in editors, running only
under CI (the `CI` env var GitHub Actions sets). Pure eslint + git — no token —
so it works on fork PRs too.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: talissoncosta

.github/workflows/frontend-pull-request.yml

@@ -58,18 +58,9 @@ jobs:
       - name: Install dependencies
         run: npm ci
 
-      # Diff-scoped: the frontend has a large pre-existing eslint/tsc backlog,
-      # so a whole-repo gate isn't viable yet.
-      - name: Lint changed files
+      # Lints only the lines this PR changed (eslint-plugin-diff), so the
+      # pre-existing backlog doesn't fail PRs. Fork-safe: pure eslint + git.
+      - name: Lint changed lines
         env:
-          BASE_SHA: ${{ github.event.pull_request.base.sha }}
-        run: |
-          CHANGED=$(git diff --name-only --diff-filter=ACMR --relative \
-            "$BASE_SHA" HEAD -- '*.ts' '*.tsx' '*.js')
-          if [ -z "$CHANGED" ]; then
-            echo "No frontend JS/TS files changed — skipping."
-            exit 0
-          fi
-          echo "Linting changed files:"
-          echo "$CHANGED"
-          echo "$CHANGED" | xargs npx eslint
+          ESLINT_PLUGIN_DIFF_COMMIT: ${{ github.event.pull_request.base.sha }}
+        run: npm run lint

frontend/.eslintrc.js

@@ -11,6 +11,9 @@ module.exports = {
     'plugin:prettier/recommended',
     'plugin:@dword-design/import-alias/recommended',
     'plugin:storybook/recommended',
+    // Lints only changed lines in CI (no-op locally), so the pre-existing
+    // eslint backlog doesn't fail PRs that merely touch a file.
+    'plugin:diff/ci',
   ],
   'globals': {
     '$': true,

frontend/package-lock.json

@@ -161,6 +161,7 @@
     "chromatic": "^15.2.0",
     "eslint": "^8.0.1",
     "eslint-config-prettier": "^8.3.0",
+    "eslint-plugin-diff": "^2.1.7",
     "eslint-plugin-import": "2.27.5",
     "eslint-plugin-import-alias": "^1.2.0",
     "eslint-plugin-prettier": "^3.4.0",