refactor: rebuild as an emblem plugin on nemo-repo-core

Everything that was not about drawing emblems moves out: the columns, the properties tab and the context menu are their own plugins now, and the config, git and cache layers come from nemo-repo-core.

The indicator list stops being hardcoded. Entries that are not one of the three git derived indicators are matched by marker_directory or marker_file, which is how claude_dir now works and how a user adds their own without touching the code.

.gitignore

@@ -0,0 +1,2 @@
+__pycache__/
+*.py[cod]