ci: centralize Slack webhook onto org-level SLACK_WEBHOOK_OSS_ALERTS (#78)

## Summary

Standardizes this repo onto the CopilotKit **org-level** secret
`SLACK_WEBHOOK_OSS_ALERTS` (visibility `all`, posts to **#oss-alerts**),
removing reliance on the repo-local bare `SLACK_WEBHOOK` secret.

This is a **pure secret-NAME swap** — the repo-local `SLACK_WEBHOOK`
value is the same #oss-alerts webhook, so there is **no behavior
change** and **no channel change**.

## Change

- `.github/workflows/notify-pr.yml`: `${{ secrets.SLACK_WEBHOOK }}` →
`${{ secrets.SLACK_WEBHOOK_OSS_ALERTS }}` (1 line). The `SLACK_WEBHOOK`
env-var alias name is retained, so the rest of the step's script is
untouched.

`publish.yml` already references `secrets.SLACK_WEBHOOK_OSS_ALERTS` and
was intentionally left unchanged.

## Validation

- `python3 -c "import yaml; yaml.safe_load(...)"` — valid
- `actionlint` — pass
- No bare `secrets.SLACK_WEBHOOK` references remain in
`.github/workflows/`

## Follow-up

The now-unused repo-local `SLACK_WEBHOOK` secret will be deleted after
merge.

Author: jpr5

.github/workflows/notify-pr.yml

@@ -10,7 +10,7 @@ jobs:
       - name: Notify Slack
         if: github.actor != 'github-actions[bot]'
         env:
-          SLACK_WEBHOOK: ${{ secrets.SLACK_WEBHOOK }}
+          SLACK_WEBHOOK: ${{ secrets.SLACK_WEBHOOK_OSS_ALERTS }}
           PR_TITLE: ${{ github.event.pull_request.title }}
           PR_URL: ${{ github.event.pull_request.html_url }}
           PR_AUTHOR: ${{ github.event.pull_request.user.login }}