chore: stop tracking sidecar JAR (build artifact)

make uatu copies the real fat JAR into assets/ before
go build -tags withsidecar. Keeping that path tracked was
the root cause of the 130 MB push rejection.

.gitignore

@@ -26,3 +26,7 @@ node_modules/
 # Secrets
 .env
 .env.local
+
+# Sidecar fat JAR — build artifact copied in by `make uatu` before
+# `go build -tags withsidecar`. Never commit: it's ~130 MB.
+internal/sidecar/assets/sidecar-all.jar