stop plugin clones being committable, and undo the symlinks that were

Merging the workspaces branch replaced `plugins/{example,music,offscale}` — real git
clones — with symlinks pointing at themselves. The build then failed with
`SymLinkLoop`, which is a confusing way to be told your plugins are gone.

My fault, in a way worth writing down. I symlinked the plugins into a git worktree so
it could build against them, then committed with `git add -A`. The links were untracked
rather than ignored, so they went in. On master, checking out a symlink where a
directory stands removes the directory — and since the clone contents were untracked,
git did it without complaint.

Recovered by re-cloning all three from gitea. Nothing was lost that was not already
pushed there; uncommitted work inside those clones would have been, which is the part
that could have been expensive.

`plugins/*` is now gitignored with `!plugins/*.md`, so the convention that plugin
clones are not this repository's content is enforced rather than remembered. A worktree
symlink cannot be committed again.

Verified after recovery: tsgo clean, frontend rebuilds, the live app serves, and
`/api/plugins` answers 200.

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

plugins/example

@@ -1 +0,0 @@
-/home/pastilhas/officer-dev/platform/plugins/example

plugins/music

@@ -1 +0,0 @@
-/home/pastilhas/officer-dev/platform/plugins/music

plugins/offscale

@@ -1 +0,0 @@
-/home/pastilhas/officer-dev/platform/plugins/offscale