feat: use the vllnt logo as the Claude plugin icon (0.1.0-canary.4)

[bntvllnt]

Summary

• Replaces the drafted stack icon with the vllnt logo, stored as assets/vllnt-logo.png (460×460 PNG).
• The file is the local copy from ~/.convex-submit/vllnt-logo.png. Downloading https://vllnt.com/vllnt-logo.png directly was not permitted, so byte equality with the website file is unverified.
• The generator wraps the PNG in .claude-plugin/icon.svg, the path the Claude directory portal asks for. It rejects inputs that aren't a PNG, aren't square, or are smaller than 128px.
• Bumps to 0.1.0-canary.4. Updates the CHANGELOG and the README asset row.

Verification

• 38 tests OK. New checks: the SVG wrapper is used, and non-square, small, or non-PNG inputs are rejected.
• sync.py --check: 413 files, no drift. git diff --check clean.
• claude plugin validate passes. This check ignores unknown manifest fields, so the SVG path was chosen over an unverified icon field.
• Rendered the generated icon.svg in headless Chromium at 256, 128, 64 and 32px on light and dark backgrounds.

https://claude.ai/code/session_01QibzbuYP9NMPNKJdURfxfE