add umdp3 fixer action

.github/workflows/umdp3_fixer.yaml

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+---
+name: Fix UMDP3
+
+on:
+  workflow_call:
+    inputs:
+      runner:
+        description: 'The runner to use for the job'
+        required: false
+        type: string
+        default: 'ubuntu-24.04'
+      timeout:
+        description: 'The time limit for the workflow, default 10 minutes'
+        required: false
+        type: number
+        default: 10
+
+permissions: read-all
+
+jobs:
+  umdp3_fixer:
+    runs-on: ${{ inputs.runner }}
+    timeout-minutes: ${{ inputs.timeout }}
+
+    steps:
+      - name: Checkout Branch
+        uses: actions/checkout@v4
+        with:
+          path: pr_branch
+          token: ${{ secrets.GITHUB_TOKEN }}
+      - name: Checkout SimSys_Scripts
+        uses: actions/checkout@v4
+        with:
+          repository: MetOffice/SimSys_Scripts
+          sparse-checkout: umdp3_fixer
+          path: SimSys_Scripts
+      - name: Set up Python 3.14
+        uses: actions/setup-python@v5
+        with:
+          python-version: 3.14
+      - name: Run UMDP3 Fixer
+        working-directory: ./SimSys_Scripts/umdp3_fixer
+        run: |
+          python rosestem_branch_checker.py --source "${GITHUB_WORKSPACE}/pr_branch" --fixer_source . --col 80

umdp3_fixer_action/README.md

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+# UMDP3 Fixer Action
+
+This action runs the UMDP3 fixer script stored in
+[MetOffice/SimSys_Scripts](https://github.com/MetOffice/SimSys_Scripts).
+
+## Usage
+
+```yaml
+name: umdp3 Fixer
+
+on:
+    pull_request:
+        types: [opened, synchronize, reopened]
+    workflow_dispatch:
+
+jobs:
+    umdp3_fixer:
+        uses: MetOffice/growss/.github/workflows/umdp3_fixer.yaml@main
+
+        # Optional
+        with:
+            runner: 'ubuntu-24.04'
+            timeout: 10
+```