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.

rootfs/etc/xdg/nvim/sysinit.vim

@@ -0,0 +1,18 @@
+" pi-devbox — system-wide Neovim defaults.
+"
+" This is Neovim's *system vimrc*: it loads for every user before any personal
+" ~/.config/nvim, and personal configs can still override it.
+"
+" Enable 24-bit ("true") colour. Without it, Neovim's default theme is squeezed
+" into a 256-colour palette where strings/comments become a muddy, low-contrast
+" dark colour — a common complaint over ssh/kitty where COLORTERM often isn't
+" propagated into the container. Modern terminals (kitty, WezTerm, iTerm2,
+" Alacritty, ...) all support true colour; the bundled kitty-terminfo also lets
+" Neovim auto-detect it, but forcing it here guarantees readable colour
+" regardless of how the terminal type / COLORTERM reach the container.
+"
+"   Opt out for a session:  :set notermguicolors
+"   Override permanently:   set your own value in ~/.config/nvim/init.lua
+if has('termguicolors')
+  set termguicolors
+endif