[chore]: add delete-outdated-pr-branches GitHub Action (#1108)

### What does this PR do?

Type of change: new feature

Adds a scheduled GitHub Action that automatically deletes
`pull-request/<num>` branches where the corresponding PR is no longer
open (i.e., closed or merged). This prevents stale branches from
accumulating over time.

### Testing

The workflow can be triggered on-demand via `workflow_dispatch` for
manual testing before relying on the weekly schedule.

### Before your PR is "*Ready for review*"

- Is this change backward compatible?: N/A
- If you copied code from any other sources or added a new PIP
dependency, did you follow guidance in `CONTRIBUTING.md`: N/A
- Did you write any new necessary tests?: N/A
- Did you update
[Changelog](https://github.com/NVIDIA/Model-Optimizer/blob/main/CHANGELOG.rst)?:
N/A

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Chores**
* Added automated workflow to periodically clean up branches associated
with closed or merged pull requests, running weekly and available for
manual trigger.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Signed-off-by: Keval Morabia <28916987+kevalmorabia97@users.noreply.github.com>

Author: kevalmorabia97

.github/workflows/delete_outdated_pr_branches.yml

@@ -0,0 +1,47 @@
+name: Delete Outdated PR Branches
+
+on:
+  schedule:
+    - cron: "0 9 * * 1" # Every Monday at 9:00 UTC
+  workflow_dispatch: # On-demand
+
+permissions:
+  contents: write
+  pull-requests: read
+
+jobs:
+  delete-outdated-pr-branches:
+    runs-on: ubuntu-latest
+    timeout-minutes: 15
+    steps:
+      - uses: actions/checkout@v6
+        with:
+          fetch-depth: 0
+      - name: Delete branches for closed/merged PRs
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        run: |
+          REPO="${{ github.repository }}"
+          DELETED=0
+          SKIPPED=0
+
+          # List all remote branches matching pull-request/<num>
+          git fetch --prune origin
+          for branch in $(git branch -r | grep -oP 'origin/pull-request/\K[0-9]+' | sort -un); do
+            FULL_BRANCH="pull-request/${branch}"
+            STATE=$(gh pr view "$branch" --repo "$REPO" --json state --jq '.state' 2>/dev/null || echo "")
+
+            if [ "$STATE" = "CLOSED" ] || [ "$STATE" = "MERGED" ]; then
+              echo "Deleting branch '${FULL_BRANCH}' (PR #${branch} is ${STATE})"
+              git push origin --delete "$FULL_BRANCH" && DELETED=$((DELETED + 1)) || true
+            elif [ "$STATE" = "OPEN" ]; then
+              echo "Skipping branch '${FULL_BRANCH}' (PR #${branch} is still OPEN)"
+              SKIPPED=$((SKIPPED + 1))
+            else
+              echo "Skipping branch '${FULL_BRANCH}' (could not determine PR #${branch} state)"
+              SKIPPED=$((SKIPPED + 1))
+            fi
+          done
+
+          echo ""
+          echo "Done. Deleted: ${DELETED}, Skipped: ${SKIPPED}"