untrack marketplace-installed plugins

The previous commit's `git add -A` committed plugins/music as a gitlink (mode
160000) — a pointer to a commit in a repository this one does not contain.
Anyone cloning the platform would get an empty directory and a broken checkout
if they could not reach gitea.

That is not a slip to be careful about next time: it is the normal consequence
of the marketplace, which puts a clone with its own .git inside this tree on
every install. So it is ignored by rule rather than by discipline.

/plugins/*/ with the shipped ones re-included by name. Adding an in-tree plugin
is deliberate, so maintaining that list by hand is the right friction — an
unlisted directory in plugins/ is an install, not source.

Verified: music ignored, example and offscale still tracked with all their files.

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

.gitignore

@@ -93,3 +93,17 @@ src/databases/officer_db/src/plugin-schemas.gen.ts
 #
 # Generate one locally whenever a diff is useful to read. It stays local.
 src/databases/officer_db/migrations/
+
+# ── Marketplace-installed plugins ──
+#
+# A plugin fetched from the marketplace is a CLONE with its own .git and its own repository. Left
+# tracked, `git add -A` after an install commits it as a gitlink — a pointer to a commit this repo does
+# not contain, which then breaks anyone who clones the platform and does not have access to that plugin.
+# Happened on 2026-08-15, one commit after the marketplace landed.
+#
+# The plugins that genuinely ship with the platform are re-included by name below. Adding one is a
+# deliberate act, so keeping this list by hand is the right amount of friction — an unlisted plugin
+# directory is an install, not a source file.
+/plugins/*/
+!/plugins/example/
+!/plugins/offscale/