assets: resize social-preview.png to 1280x640 (742 KB) for GitHub's 1MB upload limit

GitHub's Settings → Social preview upload rejects images >1 MB, and the
1920x1080 16:9 banner landed at 1.85 MB. Center-crop the vertical slab
to 2:1 (GitHub's recommended 1280x640 display size, which also means
GitHub won't re-crop server-side), resize with Lanczos, and save as
PNG at compress_level=9 + optimize=True. Result: 742 KB, under the
limit with headroom.

Kept the uncropped 1920x1080 master as assets/social-preview-2k.png so
future use cases that want the full 16:9 framing (Twitter header, blog
hero) have the high-res source. README and the upload-to-GitHub flow
both point at assets/social-preview.png (the 1280x640).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: jeffhsu-chiuchau