Use Claude Opus 4.6 for weekly stable updates agent (#1389)

Switch from 'gh issue edit --add-assignee @copilot' to the REST API
with agent_assignment to specify Claude Opus 4.6 as the model. The
simple gh CLI assignee approach doesn't support model selection,
causing the agent to default to Auto (which picks Sonnet).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: jonathanpeppers

.github/workflows/weekly-stable-updates.yml

@@ -74,8 +74,19 @@ jobs:
           console.log(`Created issue #${response.data.number}: ${response.data.title}`);
           core.setOutput('issue-number', response.data.number);
 
-    - name: Add assignee to issue
+    - name: Assign issue to Copilot with Claude Opus 4.6
       if: steps.create-issue.outputs.issue-number && steps.create-issue.outputs.issue-number != ''
-      run: gh issue edit ${{ steps.create-issue.outputs.issue-number }} --add-assignee "@copilot" -R dotnet/android-libraries
+      run: |
+        gh api \
+          --method POST \
+          -H "Accept: application/vnd.github+json" \
+          -H "X-GitHub-Api-Version: 2022-11-28" \
+          /repos/${{ github.repository }}/issues/${{ steps.create-issue.outputs.issue-number }}/assignees \
+          --input - <<< '{
+          "assignees": ["copilot-swe-agent[bot]"],
+          "agent_assignment": {
+            "model": "claude-opus-4.6"
+          }
+        }'
       env:
         GH_TOKEN: ${{ secrets.ANDROID_TEAM_PAT }}