build.sh's workspace/ is generated output, not a source change

The clean-tree gate added minutes ago immediately caught its own build: build.sh creates its
scratch directory beside the script, so a build run from a CLONE — which is exactly what the gate
pushes you to when the shared tree is busy — dirties the tree it is checking. It is generated
output and belongs in .gitignore.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0117kRTSgs3rJWUFGxyoWN6Y

controller/.gitignore

@@ -41,4 +41,8 @@ restic-password
 # App assets (synced from felhom.eu website at build time)
 assets/*.svg
 assets/*.png
-assets/*.webp
+assets/*.webp
+# build.sh's scratch directory. It is created beside the script, so a build run from a CLONE of
+# this repo (which is what the clean-tree gate pushes you to when the shared tree is busy) would
+# otherwise dirty the very tree the gate is checking. Generated output, never committed.
+workspace/