refactor: rebuild as a property page plugin on nemo-repo-core

Keeps only the PropertyPageProvider from the single file extension; the config, git and cache layers come from nemo-repo-core.

Opening Properties is an explicit request for current state, so the page now bypasses the shared TTL cache and asks git directly. The grid gains a summary row and the row builder is split out so the page body reads as a list of fields.

.gitignore

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