Add velith plugin (remote source)

[epicsagas]

What this PR does

• Plugin name: velith
• Type: remote source
• Source URL + pinned SHA: https://github.com/epicsagas/Velith.git @ 918bf4fc29d89b5e18a895f522e7407672a23be2 (tag: v0.6.0)
• Homepage: https://github.com/epicsagas/Velith

AI-native book publishing pipeline: 17 skills and 8 agents covering the full author workflow (ideation, outline architecture, chapter drafting with continuity checks, style editing, EPUB/PDF export, cover concepts). Ships a 6-phase pipeline router plus a Svelte dashboard for progress tracking.

Ownership

• I own this plugin or have the right to distribute it.
• The source repo is published under our official org (or I've explained why not below).

Checklist

• Added/updated exactly one entry in .grok-plugin/marketplace.json (valid JSON, kebab-case name).
• Remote source pins a full 40-char lowercase commit sha, and that commit is public + reachable.
• Regenerated .grok-plugin/plugin-index.json (python3 scripts/generate-plugin-index.py).
• python3 scripts/validate-catalog.py passes locally.
• Plugin is licensed (Apache-2.0, LICENSE in repo root).
• Keywords are brand-scoped (velith, velith book, velith publishing, velith epub); no own domains, so domains omitted.