docs(readme): add ShellTime logo header

[AnnatarHe]

Summary

Neovim plugins have nowhere to show an icon: no marketplace listing and no statusline glyph in this plugin. So the new ShellTime logo from shelltime/web#657 goes in the README instead.

• assets/logo.svg: a verbatim copy of the web repo's src/assets/logo/logo.github.svg.
• README.md: shows the logo centered above the # shelltime.nvim title.

No Lua code changes.

🤖 Generated with Claude Code

https://claude.ai/code/session_01GRR68kM5sbWqBV1aHyeyug

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[claude]

Review: docs-only change (README header + assets/logo.svg). No Lua changes, so no test or performance impact.

Looks good

• The SVG is clean: no scripts, external references or embedded data. Nothing to worry about security-wise.
• The relative path assets/logo.svg renders on GitHub. The width/height on the <img> keep the 1000px SVG at 96px.

Minor suggestions (non-blocking)

1. The logo has an opaque light background (#f8fafc to #e0f2fe), so it will show as a light square in GitHub dark mode. If that looks off, use a transparent or rounded-rect background, or a <picture> with a dark variant.
2. The SVG is 1000x1000 with a 256 viewBox. This works, but a smaller intrinsic size (e.g. 256) is tidier, though the README already constrains it. Since the file is meant to be a verbatim copy, I'd leave it.
3. If the logo is also meant to be a repo social preview, GitHub needs a PNG/JPG, because it doesn't accept SVG there.

LGTM.