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.

README.md

@@ -59,6 +59,12 @@ comfort level — if you'd rather not use a vi-style editor, `nano` and `micro`
 are both non-modal. Set your preference with `export EDITOR=micro` (or `nano`)
 in your shell profile, and/or `git config --global core.editor micro`.
 
+Neovim ships with a system-wide default (`/etc/xdg/nvim/sysinit.vim`) that turns
+on `termguicolors`, so its colours render in 24-bit instead of a muddy
+256-colour fallback over ssh/kitty. The `kitty-terminfo` entry is also bundled
+so `TERM=xterm-kitty` is understood. Override either in your own
+`~/.config/nvim`.
+
 ### Document and image tooling
 
 - `pandoc` — universal Markdown↔HTML/Org/RST/etc. converter