refactor: rebuild as a column plugin on nemo-repo-core

Keeps only the ColumnProvider from the single file extension; the config, git and cache layers come from nemo-repo-core, so the columns share one git query with the other plugins in the family.

The status string moves to the shared RepoInfo.summary and is now plain ASCII: 2 ahead, 1 behind instead of arrow glyphs. Both columns can be turned off in the config.

uninstall.sh

@@ -0,0 +1,20 @@
+#!/usr/bin/env bash
+# Removes this plugin from Nemo. The core, the other plugins and the config
+# file are left alone.
+set -euo pipefail
+
+dataHome="${XDG_DATA_HOME:-$HOME/.local/share}"
+extensionsDir="$dataHome/nemo-python/extensions"
+target="$extensionsDir/nemoRepoColumns.py"
+
+if [ -e "$target" ]; then
+  rm -f "$target"
+  echo "Removed $target"
+else
+  echo "Not installed: $target"
+fi
+
+rm -f "$extensionsDir/__pycache__/nemoRepoColumns".*.pyc
+
+echo
+echo "Restart Nemo to unload it:  nemo -q"