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.

Dockerfile.base

@@ -70,6 +70,12 @@ ENV DEBIAN_FRONTEND=noninteractive
 #                  htop/tmux, so it adds no extra packages. Companion to nvim
 #                  and the `micro` binary installed further down. EDITOR stays
 #                  nvim; users opt in via `export EDITOR=nano`.
+#   kitty-terminfo — terminfo entry for the kitty terminal (TERM=xterm-kitty).
+#                  ~77 KB, terminfo file only (no kitty binary). Without it,
+#                  ncurses apps fall back and Neovim can't reliably detect
+#                  true-colour from kitty over ssh; installing it makes
+#                  TERM=xterm-kitty understood. Pairs with the system-wide
+#                  Neovim termguicolors default (etc/xdg/nvim/sysinit.vim).
 RUN apt-get update && \
     apt-get upgrade -y --no-install-recommends && \
     apt-get install -y --no-install-recommends \
@@ -107,6 +113,7 @@ RUN apt-get update && \
     imagemagick \
     socat \
     nano \
+    kitty-terminfo \
     && ln -s /usr/bin/fdfind /usr/local/bin/fd \
     && apt-get clean \
     && rm -rf /var/lib/apt/lists/*
@@ -429,6 +436,11 @@ ENV LANG=en_US.UTF-8
 ENV LANGUAGE=en_US:en
 ENV LC_ALL=en_US.UTF-8
 ENV EDITOR=nvim
+# Advertise 24-bit colour so colour-aware tools (Neovim's own auto-detect, bat,
+# delta, ...) use true colour instead of a 256-colour fallback. Safe for the
+# modern terminals this devbox targets; override by exporting `COLORTERM=`
+# (empty) from a terminal that lacks true-colour support.
+ENV COLORTERM=truecolor
 ENV PATH="/home/developer/.local/bin:/home/developer/.cargo/bin:${PATH}"
 
 # ── Node.js (required for pi + MCP servers + tldr) ──
@@ -558,6 +570,13 @@ COPY rootfs/home/developer/.bash_aliases     /etc/skel-devbox/.bash_aliases
 COPY rootfs/home/developer/.inputrc          /etc/skel-devbox/.inputrc
 COPY rootfs/home/developer/.gitignore_global /etc/skel-devbox/.gitignore_global
 
+# ── Editor defaults: system-wide Neovim true-colour ──────────────────
+# /etc/xdg/nvim/sysinit.vim is Neovim's system vimrc: it loads for every user
+# (before any personal ~/.config/nvim) and can still be overridden per-user.
+# Enables termguicolors so the default theme renders in 24-bit colour instead
+# of a muddy 256-colour fallback. Pairs with kitty-terminfo (installed above).
+COPY rootfs/etc/xdg/nvim/sysinit.vim /etc/xdg/nvim/sysinit.vim
+
 # ── Entrypoint ────────────────────────────────────────────────────────
 COPY rootfs/usr/local/lib/pi-devbox/ /usr/local/lib/pi-devbox/
 # Image-baked skills + the global-AGENTS append snippet. Under /usr/local so a