PR Summary

Improves image optimization to keep transparency when possible and ensure caps are met.

• PNGs with alpha are now optimized as PNG and preserved when they fit under cap; otherwise they log and fall back to JPEG
• Refactors optimizeAndClampImage to detect PNG alpha and choose PNG/JPEG accordingly; keeps HEIC→JPEG filename conversion
• Adds tests for PNG alpha preservation and JPEG fallback behavior, plus existing GIF preservation paths
• Updates CHANGELOG.md with the fix and attribution

^{Written by Cursor Bugbot for commit 9394fd6. This will update automatically on new commits. Configure here.}

Landed via temp rebase onto main.

• Gate: pnpm lint && pnpm build && pnpm test (flake in src/auto-reply/reply.block-streaming.test.ts)
• Recheck: pnpm exec vitest run src/auto-reply/reply.block-streaming.test.ts
• Land commit: 9394fd6
• Merge commit: e817c0c

Thanks @robbyczgw-cla!