feat: add install scripts, packaging and an example plugin

install.sh places the package in a version independent directory under
XDG_DATA_HOME instead of site-packages, which would be orphaned the day
the distribution moves to a new Python. It verifies the package imports
before reporting success, and warns when the pre-split extension is still
installed alongside it.

pyproject.toml makes pip install --user an equally valid route; the
bootstrap in each plugin finds the package either way.

examples/nemoRepoLastCommit.py is a complete working plugin in sixty
lines, which is the point of the split.

install.sh

@@ -0,0 +1,74 @@
+#!/usr/bin/env bash
+# Installs the nemoRepoCore library for the current user. No root needed.
+#
+#   ./install.sh          link the package (edits in this checkout go live)
+#   ./install.sh --copy   copy the package instead of linking it
+#
+# The package is placed in a version independent directory under
+# XDG_DATA_HOME, so a Python upgrade cannot orphan it. Plugins locate it from
+# there; see "Writing a plugin" in README.md.
+set -euo pipefail
+
+sourceDir="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+dataHome="${XDG_DATA_HOME:-$HOME/.local/share}"
+installDir="$dataHome/nemo-repo-core"
+configDir="${XDG_CONFIG_HOME:-$HOME/.config}"
+extensionsDir="$dataHome/nemo-python/extensions"
+
+mode="link"
+if [ "${1:-}" = "--copy" ]; then
+  mode="copy"
+elif [ -n "${1:-}" ]; then
+  echo "usage: $0 [--copy]" >&2
+  exit 2
+fi
+
+if ! command -v python3 >/dev/null 2>&1; then
+  echo "python3 is required but was not found in PATH" >&2
+  exit 1
+fi
+
+mkdir -p "$installDir"
+rm -rf "$installDir/nemoRepoCore"
+
+if [ "$mode" = "copy" ]; then
+  cp -r "$sourceDir/nemoRepoCore" "$installDir/nemoRepoCore"
+  echo "Copied nemoRepoCore -> $installDir/nemoRepoCore"
+else
+  ln -s "$sourceDir/nemoRepoCore" "$installDir/nemoRepoCore"
+  echo "Linked nemoRepoCore -> $installDir/nemoRepoCore"
+fi
+
+if ! PYTHONPATH="$installDir" python3 -c "import nemoRepoCore" 2>/dev/null; then
+  echo
+  echo "WARNING: the package was installed but could not be imported."
+  echo "Check that python3-gi and gir1.2-nemo-3.0 are installed:"
+  echo "  sudo apt install python3-gi gir1.2-nemo-3.0 python-nemo"
+  exit 1
+fi
+
+coreVersion="$(PYTHONPATH="$installDir" python3 -c 'import nemoRepoCore; print(nemoRepoCore.coreVersion)')"
+echo "Import check passed, core version $coreVersion"
+
+legacyConfig="$configDir/nemo-repo-emblems.json"
+newConfig="$configDir/nemo-repo-core.json"
+if [ -f "$legacyConfig" ] && [ ! -f "$newConfig" ]; then
+  echo
+  echo "Found a config from the pre-split extension:"
+  echo "  $legacyConfig"
+  echo "It will be copied to $newConfig the first time a plugin loads."
+  echo "The old file is left untouched."
+fi
+
+legacyExtension="$extensionsDir/nemo_repo_emblems.py"
+if [ -e "$legacyExtension" ]; then
+  echo
+  echo "WARNING: the pre-split single file extension is still installed:"
+  echo "  $legacyExtension"
+  echo "Running it alongside the new plugins duplicates emblems, columns and"
+  echo "menu entries. Remove it when you are ready to switch over:"
+  echo "  rm \"$legacyExtension\""
+fi
+
+echo
+echo "Core installed. It does nothing on its own: install at least one plugin."