Added slack issue notifier for new opened issues (#356)

Posts new issues to claude-agent-sdk-feedback. Can't use the default
github slack bot as it's too noisy (all comments and issue updates such
as comments, closing, etc...)

---------

Co-authored-by: Ashwin Bhat <ashwin@anthropic.com>

.github/workflows/slack-issue-notification.yml

@@ -0,0 +1,36 @@
+name: Post new issues to Slack
+
+on:
+  issues:
+    types: [opened]
+
+jobs:
+  notify:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Post to Slack
+        uses: slackapi/slack-github-action@91efab103c0de0a537f72a35f6b8cda0ee76bf0a # 2.1.1
+        with:
+          method: chat.postMessage
+          token: ${{ secrets.SLACK_BOT_TOKEN }}
+          payload: |
+            {
+              "channel": "C09HY5E0K60",
+              "text": "New issue opened in ${{ github.repository }}",
+              "blocks": [
+                {
+                  "type": "section",
+                  "text": {
+                    "type": "mrkdwn",
+                    "text": "*New Issue:* <${{ github.event.issue.html_url }}|#${{ github.event.issue.number }} ${{ github.event.issue.title }}>"
+                  }
+                },
+                {
+                  "type": "section",
+                  "text": {
+                    "type": "mrkdwn",
+                    "text": "*Author:* ${{ github.event.issue.user.login }}"
+                  }
+                }
+              ]
+            }