[github-actions] add monthly CalVer release workflow (#13125)

This commit introduces a new GitHub Actions workflow to automate the
monthly release process using Calendar Versioning (CalVer).

The workflow:
- Runs automatically at 00:00 UTC on the 1st day of every month.
- Supports manual execution via `workflow_dispatch`.
- Automatically generates a CalVer tag (e.g., vYYYY.MM.0).
- Employs the GitHub CLI to create a release and auto-generate
  release notes based on merged pull requests.

.github/release.yml

@@ -0,0 +1,49 @@
+#
+#  Copyright (c) 2026, The OpenThread Authors.
+#  All rights reserved.
+#
+#  Redistribution and use in source and binary forms, with or without
+#  modification, are permitted provided that the following conditions are met:
+#  1. Redistributions of source code must retain the above copyright
+#     notice, this list of conditions and the following disclaimer.
+#  2. Redistributions in binary form must reproduce the above copyright
+#     notice, this list of conditions and the following disclaimer in the
+#     documentation and/or other materials provided with the distribution.
+#  3. Neither the name of the copyright holder nor the
+#     names of its contributors may be used to endorse or promote products
+#     derived from this software without specific prior written permission.
+#
+#  THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+#  AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+#  IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+#  ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
+#  LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+#  CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+#  SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+#  INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+#  CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+#  ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+#  POSSIBILITY OF SUCH DAMAGE.
+#
+
+changelog:
+  exclude:
+    labels:
+      - duplicate
+      - question
+      - invalid
+      - wontfix
+    authors:
+      - dependabot[bot]
+  categories:
+    - title: "🚀 Features"
+      labels:
+        - feature
+        - enhancement
+    - title: "🐛 Bug Fixes"
+      labels:
+        - bug
+        - fix
+    - title: "📖 Documentation"
+      labels:
+        - documentation