fix: prevent duplicate website-e2e CI runs on PRs (#11607)

## Summary

Fix duplicate `CI: Website E2E` workflow runs on pull requests.

## Problem

Two runs were triggered for every PR touching website files:
- `website-e2e (pull_request)` — from the PR event
- `website-e2e (push)` — from the push to a `website/*` branch

The concurrency key used `github.ref`, which evaluates differently for
push (`refs/heads/...`) vs pull_request (`refs/pull/N/merge`), so they
couldn't cancel each other.

## Changes

1. Scope `push` trigger to `main` only (removes `website/*`)
2. Use `github.head_ref || github.ref` in the concurrency group so push
and PR events for the same branch share a group

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-11607-fix-prevent-duplicate-website-e2e-CI-runs-on-PRs-34c6d73d3650814c9d24c77b1591e94a)
by [Unito](https://www.unito.io)

---------

Co-authored-by: Amp <amp@ampcode.com>

.github/workflows/ci-website-e2e.yaml

@@ -2,7 +2,7 @@ name: 'CI: Website E2E'
 
 on:
   push:
-    branches: [main, website/*]
+    branches: [main]
     paths:
       - 'apps/website/**'
       - 'packages/design-system/**'
@@ -17,7 +17,7 @@ on:
       - 'pnpm-lock.yaml'
 
 concurrency:
-  group: ${{ github.workflow }}-${{ github.ref }}
+  group: ${{ github.workflow }}-${{ github.repository }}-${{ github.head_ref || github.ref }}
   cancel-in-progress: true
 
 jobs: