Add auto-author-assign workflow (valkey-io#2410)

Fixes valkey-io#2372

## Description

This PR adds an automated workflow that assigns PR authors to their own
pull requests when opened or reopened.

## Changes

- Added `.github/workflows/auto-author-assign.yml` workflow
- Uses `toshimaru/auto-author-assign@v2.1.1` action
- Triggers on `pull_request_target` events (opened, reopened)
- Requires `pull-requests: write` permission

## Benefits

- Improves PR tracking and organization
- Automatically assigns responsibility to PR authors
- Reduces manual assignment overhead for maintainers
- Helps contributors track their own PRs more easily

## Testing

✅ **Tested on my fork before submission:**

1. Merged the workflow into my fork's unstable branch
2. Created a test PR within my fork
3. Verified that I was automatically assigned as the assignee
4. Screenshot showing successful auto-assignment:
<img width="1278" height="684" alt="Screenshot 2025-08-01 at 3 39 05 PM"
src="https://github.com/user-attachments/assets/9ad643be-5eac-4ad6-bec7-184cf22e9cbd"
/>

The workflow executed successfully and assigned me to the test PR as
expected.

---------

Signed-off-by: Hanxi Zhang <hanxizh@amazon.com>

Author: hanxizh9910

.github/workflows/auto-author-assign.yml

@@ -0,0 +1,14 @@
+name: Auto Author Assign
+
+on:
+  pull_request_target:
+    types: [opened, reopened]
+
+permissions:
+  pull-requests: write
+
+jobs:
+  assign-author:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: toshimaru/auto-author-assign@16f0022cf3d7970c106d8d1105f75a1165edb516 # v2.1.1