portcullis: Vim stops stealing the terminal's mouse selection

portcullis hardens a fresh Ubuntu host, and hardening has to leave the machine pleasant to actually use: Ubuntu’s Vim defaults enable xterm mouse reporting, so on a remote terminal Vim captures the selection and ordinary copy/paste stops working. A new Phase 2 section, 1.6a — Terminal editor defaults, now ships /etc/vim/vimrc.local with set mouse= (and a note in the README’s Phase 2 list), leaving selection to the terminal emulator while anyone who wants Vim’s mouse support can opt back in.

The placement is the point: Vim sources vimrc.local before its own defaults.vim, so doing it there alone would have been overwritten. The override is deferred to a VimEnter autocmd inside a dedicated augroup instead, and the same commit adds the repository’s first Phase 2 test, tests/test_phase2_defaults.py, which asserts both the vimrc.local write and the deferred VimEnter line so the fix cannot silently regress.