How to fix contacts being read-only?
It looks like there is, as you mention, a file permission problem, or maybe the Contacts database is corrupt.

In either case, try rebuilding the database:


Open Contacts and export your contacts with File&gt;Export...&gt;Contacts Archive....
Quit Contacts.
Open ~/Library/Application Support/ in the Finder (you will need to switch to the Finder, press Shift⌘G, type ~/Library/Application Support and press Enter).
Rename ~/Library/Application Support/AddressBook to ~/Library/Application Support/AddressBook.bak.
Start Contacts. You will notice that there are only two contacts: Apple and you. Delete them.
Import the previously exported contacts with File&gt;Import....