Fix git checkout error when switching to pr-screenshot branch

The script was failing with "Your local changes to the following files would be overwritten by checkout" because:
- Screenshot tests run with --update-goldens, which modifies files in test/screenshots/goldens/
- These files are then copied to a temp directory
- When attempting to checkout pr-screenshot branch, git refuses due to uncommitted local changes

Fixed by using `git checkout -f` (force) to discard local changes when switching branches. This is safe because:
- Screenshots are already copied to temp directory before checkout
- The modified golden files are test artifacts, not production code
- We want to switch to the clean pr-screenshot branch regardless of local changes

This allows the workflow to successfully commit screenshots to the pr-screenshot branch.

Co-authored-by: richardthe3rd <573334+richardthe3rd@users.noreply.github.com>

Author: Copilot

scripts/update_screenshot_branch.sh

@@ -42,7 +42,10 @@ git config --local user.name "github-actions[bot]"
 if git ls-remote --heads origin "$BRANCH_NAME" | grep -q "$BRANCH_NAME"; then
   echo "Branch $BRANCH_NAME exists, checking it out..."
   git fetch origin "$BRANCH_NAME"
-  git checkout "$BRANCH_NAME"
+  
+  # Force checkout to discard local changes in test/screenshots/goldens/
+  # (screenshots are already copied to temp directory)
+  git checkout -f "$BRANCH_NAME"
 
   # Pull latest changes
   git pull origin "$BRANCH_NAME"