Remove stray empty files; whitelist the repo layout in .gitignore

Three zero-byte shell-redirection accidents were tracked. The
.gitignore now ignores everything at the root except the known data
directories, README, and workflows, so strays can never be committed
again even with git add -A.

.gitignore

@@ -0,0 +1,9 @@
+# Whitelist: this repo holds ONLY measurement data with a fixed layout.
+# Everything at the root is ignored unless explicitly allowed below, so stray
+# files (shell redirection accidents, tool artifacts) can never be committed.
+/*
+!/.gitignore
+!/.gitea/
+!/README.md
+!/lamps/
+!/solar-charge-controllers/