Distribute the uncov skill as a Claude Code plugin/marketplace

[llbbl]

Once the uncov skill exists (#6), make llbbl/uncov its own native Claude Code marketplace so users install with /plugin marketplace add llbbl/uncov → /plugin install <plugin>@llbbl-uncov. This gives first-class auto-update, /plugin discovery, and clean uninstall — replacing the current DIY/guide-based setup.

Background

• A plugin is defined by .claude-plugin/plugin.json (name, description, version, author) with skills under skills/<name>/SKILL.md.
• A marketplace is defined by .claude-plugin/marketplace.json cataloging the plugin(s). A single repo can be its own marketplace.
• The skill ships as pure markdown and calls uncov from PATH (no bundled binary), so the plugin stays tiny.

Acceptance

• Repo contains .claude-plugin/plugin.json and .claude-plugin/marketplace.json.
• /plugin marketplace add llbbl/uncov then /plugin install <plugin>@<marketplace> installs the uncov skill, namespaced.
• README documents the marketplace install path (and notes the uncov binary prerequisite via brew/npm/install.sh).
• Verified end-to-end on a clean profile.

Tasks

• Add .claude-plugin/plugin.json
• Add .claude-plugin/marketplace.json cataloging the uncov plugin
• Confirm skills/ layout matches plugin expectations (skills/<name>/SKILL.md)
• Document marketplace install in README
• Verify /plugin marketplace add + /plugin install end-to-end

Depends on #6 — the skill must exist (and be PATH-based) before it can ship via the marketplace.