ci: auto-label PRs by size via cbrgm/pr-size-labeler-action (#1101)

Adds a GitHub Actions workflow and config that labels every PR with
size/xs..size/xl based on files changed and lines added, so reviewers
can see at a glance how big a PR is. Lock files and generated/vendored
paths are excluded, and deletions aren't counted so refactors that
remove code aren't over-penalized.

Co-authored-by: Orca <help@stably.ai>

.github/pull-request-size.yml

@@ -0,0 +1,50 @@
+# Configuration for cbrgm/pr-size-labeler-action
+# See: https://github.com/cbrgm/pr-size-labeler-action
+#
+# A PR gets the smallest label whose `diff` and `files` thresholds both fit.
+# Generated/lock/vendored files are excluded so they don't dominate the size calculation.
+
+exclude_files:
+  - "**/pnpm-lock.yaml"
+  - "**/package-lock.json"
+  - "**/yarn.lock"
+  - "**/*.lock"
+  - "**/*.snap"
+  - "**/dist/**"
+  - "**/build/**"
+  - "**/node_modules/**"
+  - "**/vendor/**"
+  - "**/*.min.js"
+  - "**/*.min.css"
+  - "**/*.map"
+  - "**/*.generated.*"
+  - "**/generated/**"
+
+label_configs:
+  - size: xs
+    diff: 25
+    files: 1
+    labels: ["size/xs"]
+
+  - size: s
+    diff: 150
+    files: 10
+    labels: ["size/s"]
+
+  - size: m
+    diff: 600
+    files: 25
+    labels: ["size/m"]
+
+  - size: l
+    diff: 2500
+    files: 50
+    labels: ["size/l"]
+
+  - size: xl
+    diff: 5000
+    files: 100
+    labels: ["size/xl"]
+
+# Count only added lines (ignore deletions) so large refactors/removals aren't over-penalized.
+added_lines_only: true

.github/workflows/pr-size-labeler.yml

@@ -0,0 +1,29 @@
+name: PR Size Labeler
+
+on:
+  pull_request:
+    types:
+      - opened
+      - synchronize
+      - reopened
+      - edited
+
+permissions:
+  pull-requests: write
+  contents: read
+
+jobs:
+  label-pr-size:
+    runs-on: ubuntu-latest
+    timeout-minutes: 5
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Label PR based on size
+        uses: cbrgm/pr-size-labeler-action@v1.3.7
+        with:
+          github_token: ${{ secrets.GITHUB_TOKEN }}
+          github_repository: ${{ github.repository }}
+          github_pr_number: ${{ github.event.pull_request.number }}
+          config_file_path: .github/pull-request-size.yml