example leaves too; plugins/ is documentation now

Done with `tea` as admin: plugins/offscale made public, plugins/example created
public, both verified anonymously over https — which is the check that matters,
because fetch.ts clones with no credentials and a private plugin simply cannot be
installed from the marketplace.

  plugins/example    8 files, cloned back and diffed identical
  plugins/offscale   now public (was private, so was uninstallable)

`.gitignore` is one line now — `/plugins/*/`. The trailing slash is the whole
design: it ignores DIRECTORIES, so files at the top of plugins/ stay tracked. The
documentation about plugins belongs to the platform; the plugins do not.

All three are in the marketplace catalogue, which is what makes this reversible:
before this, untracking offscale meant a fresh install could never get it back.
Now the store is the way in for all three.

CatalogueEntry gains `icon` (a lucide name) alongside `iconUrl`. Both new entries
ship glyphs rather than artwork, and without it they drew the generic puzzle piece
in the store and their real icon after install — which reads as the icon changing
rather than the store not knowing it.

EXTRACTING-A-PLUGIN.md pointed at four directories a fresh checkout no longer has.
It now gives repository URLs, says up front that this directory holds documentation
rather than plugins, and carries a table of where each one went. Checked first that
no tracked source statically imports a plugin directory — none does, so a fresh
clone still builds.

tsgo clean, frontend builds, 787 pass / 7 fail (unchanged).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/servers/api/plugins/router.ts

@@ -78,6 +78,8 @@ pluginsRouter.get('/', async (ctx) => {
       label: e.label,
       summary: e.summary,
       image: e.iconUrl,
+      // Same precedence the installed list uses: artwork wins, the glyph name is the fallback.
+      ...(e.iconUrl ? {} : { icon: e.icon }),
       tile: e.tile ?? 'badge',
       color: e.color,
       publisher: e.publisher,