Fix broken screenshot references in issue proposal #25
Description
Priority: P1 | Source: PR #21 self-review (F1)
Problem
.github/ISSUE_PROPOSAL_mechanical_diagrams.md references ./assets/car-diagram-screenshot.png and ./assets/airplane-diagram-screenshot.png but the .github/assets/ directory does not exist. Images render as broken links.
Fix
Either add the screenshot assets or remove the image references from the proposal document.