test(skills): fail the build when a skill is not installable - #93

Nine of thirteen skills were unreachable until 0.37.1 — shipped, published to
npm, and listed by nobody, because /plugin marketplace add shows exactly the
entries in .claude-plugin/marketplace.json. An entry got added alongside
whichever skill prompted it and the rest were never backfilled. Nothing failed,
because nothing looked.

Guards both directions, since both are silent: a skills/<name>/SKILL.md with no
entry is invisible to users, and an entry whose directory moved or was renamed
is a broken install rather than a missing one. Also pins source == ./skills/
<name> (a mismatch installs the wrong skill under the right label, which looks
like it worked), non-empty descriptions, and name uniqueness.

Written as a test rather than a new script + CI step because npm test already
gates PRs and publish, and because brand-numbers.test.ts established the
pattern: assert the invariant against the real files in the repo that can fix
it. Nothing here is auto-fixable anyway — a marketplace description is
hand-written prose, and generating one from the frontmatter would produce worse
text than the omission it replaces.

Sibling to skill-frontmatter.test.ts, which proves a SKILL.md can LOAD. This
proves it can be REACHED. Neither implies the other. Both take "a directory
with SKILL.md at its root" as the definition of a skill — deliberately the same
definition, or one of them can pass vacuously.

Verified by breaking each invariant in turn and confirming the matching test
fails: dropped entry, entry naming a nonexistent skill, wrong source path,
empty description, duplicate name. The unregistered-skill failure names the
directories and prints the JSON to paste.

Test-only; skills/ and dist ship to npm but test/ does not, so this needs no
version bump.