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.

CHANGELOG.md

@@ -24,6 +24,15 @@ Pre-v1.0.0 tags followed the pi npm version (`v{pi_version}[letter]`).
   `~/.gitignore_global` is absent); their file can be updated by hand. Base-
   affecting (`Dockerfile.base` COPY of the seed), rebuilds `base-<hash>`.
 
+- **Readable Neovim colours out of the box.** The base now ships a system-wide
+  Neovim config (`/etc/xdg/nvim/sysinit.vim`) that enables `termguicolors`,
+  plus the `kitty-terminfo` package. Vanilla Neovim otherwise fell back to a
+  256-colour palette over ssh/kitty and rendered strings and comments in a
+  muddy, low-contrast dark colour. `sysinit.vim` is Neovim's system vimrc: it
+  loads for every user before any personal `~/.config/nvim` and can still be
+  overridden per-user (`:set notermguicolors`, or your own init). Base-affecting
+  (`Dockerfile.base` apt package + COPY), rebuilds `base-<hash>`.
+
 ### Fixed
 
 - **`pandoc --pdf-engine=typst` now works without `-V mainfont`.** pandoc's