Resolve issue #7000 by adding PR labeler workflow (#7203)

* Add PR labeler workflow with size labeling (for now)

This size labeling workflow should be a close approximation to what
CirqBot currently does via `pr_monitor`. Once in place, it should be
possible to remove pr_monitor from the Cirq code base, because the
automerge functionality of pr_monitor (which is the other thing that
it did) is now implemented using GitHub's merge queues.

This basically resolves #7000.

* Remove message for XL changes

Review comments led to the conclusion that the message is unnecessary
and more likely to be annoying than helpfu.

Author: mhucka

.github/workflows/pr-labeler.yaml

@@ -0,0 +1,61 @@
+# Copyright 2025 The Cirq Developers
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+# Label PRs with labels such as size.
+# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+name: Pull request labeler
+run-name: >-
+  Label pull request ${{github.event.pull_request.number}} by ${{github.actor}}
+
+on:
+  pull_request:
+    types: [opened]
+
+  # Allow manual invocation.
+  workflow_dispatch:
+
+# Declare default permissions as read only.
+permissions: read-all
+
+jobs:
+  label-pr-size:
+    name: Add size label to new pull request
+    runs-on: ubuntu-24.04
+    permissions:
+      contents: read
+      pull-requests: write
+      issues: write
+    steps:
+      - name: Label the PR with a size label
+        uses: codelytv/pr-size-labeler@c7a55a022747628b50f3eb5bf863b9e796b8f274 # v1
+        with:
+          # Don't count file deletions, per suggestion in Small CLs.
+          ignore_file_deletions: 'true'
+
+          xs_label: 'Size: XS'
+          xs_max_size: '10'
+
+          s_label: 'size: S'
+          s_max_size: '50'
+
+          m_label: 'size: M'
+          m_max_size: '250'
+
+          l_label: 'size: L'
+          l_max_size: '1000'
+
+          xl_label: 'size: XL'
+          fail_if_xl: 'false'