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.

examples/nemoRepoLastCommit.py

@@ -0,0 +1,75 @@
+#!/usr/bin/python3
+"""Example plugin: a "Last Commit" column for Nemo's list view.
+
+Copy this file to ~/.local/share/nemo-python/extensions/ and restart Nemo
+with `nemo -q` to try it. It is about sixty lines of real work because
+nemo-repo-core already owns the config file, the git call and the cache.
+
+The block above the imports is the core bootstrap. Copy it verbatim into
+every plugin: it makes the plugin work whether the core was installed with
+its install.sh or with pip, and it is the only piece that knows where the
+core lives.
+"""
+
+import importlib.util
+import os
+import sys
+
+
+def _ensureCoreOnPath():
+    if importlib.util.find_spec("nemoRepoCore") is not None:
+        return
+    dataHome = os.environ.get("XDG_DATA_HOME") or os.path.expanduser("~/.local/share")
+    dataDirs = (os.environ.get("XDG_DATA_DIRS") or "/usr/local/share:/usr/share").split(":")
+    for base in [dataHome] + dataDirs:
+        candidate = os.path.join(base, "nemo-repo-core")
+        if os.path.isdir(os.path.join(candidate, "nemoRepoCore")):
+            sys.path.insert(0, candidate)
+            return
+
+
+_ensureCoreOnPath()
+
+import gi
+
+gi.require_version("Nemo", "3.0")
+from gi.repository import GObject, Nemo
+
+from nemoRepoCore import (findRepoRoot, localDirectoryPath, repoCache,
+                          requireCore)
+
+requireCore("1.0.0")
+
+
+class LastCommitColumnProvider(GObject.GObject, Nemo.ColumnProvider,
+                               Nemo.InfoProvider, Nemo.NameAndDescProvider):
+
+    def get_columns(self):
+        return [
+            Nemo.Column(name="NemoRepoExample::last_commit",
+                        attribute="repo_last_commit",
+                        label="Last Commit",
+                        description="Subject and age of the most recent commit"),
+        ]
+
+    def update_file_info(self, file):
+        path = localDirectoryPath(file)
+        if path is None:
+            return
+        root = findRepoRoot(path)
+        if root is None:
+            file.add_string_attribute("repo_last_commit", "")
+            return
+
+        repoCache.registerFileObject(root, file)
+        info = repoCache.get(root)
+        if info.error or not info.lastCommit:
+            file.add_string_attribute("repo_last_commit", "")
+            return
+
+        commit = info.lastCommit
+        file.add_string_attribute("repo_last_commit",
+                                  "%s (%s)" % (commit.subject, commit.relativeDate))
+
+    def get_name_and_desc(self):
+        return ["nemo-repo-example:::Example column showing the last commit"]