ci: add workflow to ensure go.mod files are tidied (gnolang#3025)

<!-- please provide a detailed description of the changes made in this
pull request. -->

<details><summary>Contributors' checklist...</summary>

- [ ] Added new tests, or not needed, or not feasible
- [ ] Provided an example (e.g. screenshot) to aid review or the PR is
self-explanatory
- [ ] Updated the official documentation or not needed
- [ ] No breaking changes were made, or a `BREAKING CHANGE: xxx` message
was included in the description
- [ ] Added references to related issues and PRs
- [ ] Provided any useful hints for running manual tests
</details>

.github/workflows/mod-tidy.yml

@@ -0,0 +1,26 @@
+name: Ensure go.mods are tidied
+
+on:
+  push:
+    branches:
+      - master
+  workflow_dispatch:
+  pull_request:
+
+jobs:
+  main:
+    name: Ensure go.mods are tidied
+    runs-on: ubuntu-latest
+    steps:
+      - name: Install Go
+        uses: actions/setup-go@v5
+        with:
+          go-version: ${{ inputs.go-version }}
+
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Check go.mod files are up to date
+        working-directory: ${{ inputs.modulepath }}
+        run: |
+          make tidy VERIFY_GO_SUMS=true

tm2/pkg/amino/tests/proto3/proto/doc.go

@@ -0,0 +1,3 @@
+// This file ensures there is at least one go file in this dir at all times.
+
+package proto3