[codex] refine repo presentation

[ChloeVPin]

What changed

• Centered the README intro around the Qoder logo and title
• Added the Discord invite as the repo homepage and in the README
• Reworked CONTRIBUTING for a fork-first workflow
• Swapped in the new Qoder logo assets and removed the old icon files

Why

• Make the repository look intentional on GitHub
• Keep the upstream repo reference-only while letting users fork their own working copy
• Reduce direct contribution noise on the protected branch

Notes

• main is branch-protected, so changes should land through pull requests
• The repository is public and forkable