Author a Claude Code skill (skills/uncov/SKILL.md) for the coverage workflow

[llbbl]

uncov has no in-repo Claude Code skill — there is no SKILL.md anywhere in the repo. The only skill-shaped artifact is a local, non-distributed setup guide at ~/.claude/guides/uncov-setup.md. To distribute uncov's coverage workflow as a Claude Code skill (and eventually via a marketplace, like llbbl/upkeep), an actual skill needs to be authored in the repo.

The skill should wrap the existing uncov CLI (uncov default report, uncov init, uncov check) and call it from PATH — the binary is installed separately (Homebrew brew install llbbl/tap/uncov, npm @llbbl/uncov, or install.sh), never bundled into the skill.

Acceptance

• skills/uncov/SKILL.md exists with valid frontmatter (name, description, allowed-tools).
• The skill invokes uncov from PATH only — no bundled binary, no ./bin/uncov.
• It documents that the uncov binary is a prerequisite and how to get it (brew / npm / install.sh).
• It covers the core flow: ensure Vitest/Istanbul coverage JSON exists → run uncov to report low-coverage files → optionally uncov init (bootstrap) / uncov check (verify setup).
• Content is seeded/ported from ~/.claude/guides/uncov-setup.md.
• Fails gracefully with an actionable message if uncov is not on PATH.

Tasks

• Create skills/uncov/SKILL.md with frontmatter
• Port setup + usage content from ~/.claude/guides/uncov-setup.md
• Ensure every CLI call uses uncov on PATH; document the install prerequisite
• Add an "uncov not found on PATH" check/message

Prerequisite for distributing the skill via a marketplace.