base: readable Neovim colours out of the box (termguicolors + kitty-terminfo)

Vanilla Neovim fell back to a 256-colour palette over ssh/kitty and rendered
strings/comments in a muddy, low-contrast colour. Fix, all base-affecting:

- rootfs/etc/xdg/nvim/sysinit.vim: system vimrc enabling termguicolors. Loads
  for every user before any personal ~/.config/nvim; overridable per-user.
- Dockerfile.base: install kitty-terminfo (TERM=xterm-kitty understood; lets
  Neovim auto-detect true colour) and set ENV COLORTERM=truecolor.
- smoke-test.sh: assert xterm-kitty terminfo present and nvim tgc default on.
- CHANGELOG (Unreleased/Added) + README editor note.

No tag — lands on the next base-<hash> rebuild.

scripts/smoke-test.sh

@@ -76,6 +76,8 @@ run "uv"              "uv --version"
 run "nvim"            "nvim --version"
 run "nano"            "nano --version"
 run "micro"           "micro --version"
+run "kitty-terminfo"  "infocmp -x xterm-kitty >/dev/null 2>&1"
+run "nvim true-colour default (sysinit.vim)" "nvim --headless -c 'lua os.exit(vim.o.termguicolors and 0 or 1)'"
 run "mempalace-mcp"   "mempalace-mcp --help"
 # v1.0.0 base additions — verify presence and basic functionality.
 run "pandoc"          "pandoc --version"