feat: ship nano + micro (non-modal editors) alongside nvim

The image shipped only nvim (EDITOR=nvim), a modal vi-style editor. Add
both a classic and a modern non-modal option so users who aren't
comfortable with vi keybindings have a choice:

- nano (apt): ~2.8 MB installed; deps (libc6, libncursesw6, libtinfo6)
  already present via nvim/less/htop/tmux, so no extra packages pulled in.
- micro: ~12 MB single static Go binary from GitHub releases (same pattern
  as bat/eza/zoxide). Desktop-style keys (Ctrl+S/Ctrl+Q), mouse, syntax
  highlighting. ARG MICRO_VERSION pins; defaults to latest.

Combined ~15 MB (well within the smoke size threshold's ~250 MB headroom,
so no threshold bump). EDITOR stays nvim; both editors are opt-in
(export EDITOR=micro | nano). Uses the canonical micro-editor/micro URL
because the old zyedidia/micro org rename makes /releases/latest redirect
to another /latest, defeating the tag-parsing latest-resolution idiom
(independently verified: old org 302s to micro-editor/micro; both arch
tarballs HTTP 200; latest resolves to v2.0.15).

Base-image change, so it lands on the next base-<hash> rebuild. Updates
README (what's-inside tree + EDITOR note), CHANGELOG (Unreleased/Added),
and smoke-test.sh (nano + micro presence checks). Ported from pi-devbox
3a59e15.

Dockerfile.base

@@ -35,6 +35,11 @@ ENV DEBIAN_FRONTEND=noninteractive
 # apt-get upgrade picks up any security/CVE fixes published between
 # debian:trixie-slim base-image rebuilds. Paired with the index update
 # and the install in the same layer so we don't bloat image history.
+# `nano` is included as a small, non-modal terminal editor for users who
+# don't want vi-style modal editing — a companion to nvim and the `micro`
+# binary installed further down. ~2.8 MB; its deps (libc6, libncursesw6,
+# libtinfo6) are already pulled in by nvim/less/htop/tmux, so it adds no
+# extra packages. EDITOR stays nvim; opt in via `export EDITOR=nano`.
 RUN apt-get update && \
     apt-get upgrade -y --no-install-recommends && \
     apt-get install -y --no-install-recommends \
@@ -66,6 +71,7 @@ RUN apt-get update && \
     rsync \
     python3-pip \
     python3-venv \
+    nano \
     && ln -s /usr/bin/fdfind /usr/local/bin/fd \
     && apt-get clean \
     && rm -rf /var/lib/apt/lists/*
@@ -204,6 +210,33 @@ RUN ARCH=$(case "${TARGETARCH}" in amd64) echo "x86_64" ;; arm64) echo "arm64" ;
     ln -s /opt/nvim-linux-${ARCH}/bin/nvim /usr/local/bin/nvim && \
     nvim --version | head -1
 
+# micro — modern, non-modal terminal editor. Ships alongside nvim so users
+# who aren't comfortable with vi-style modal editing have a friendly option:
+# desktop-style keybindings (Ctrl+S save, Ctrl+Q quit, Ctrl+C/V/X, Ctrl+Z
+# undo), mouse support, and syntax highlighting out of the box. A single
+# static Go binary (~12 MB) installed from GitHub releases, exactly like
+# bat/eza/zoxide below. EDITOR stays nvim (see below); users opt in with
+# `export EDITOR=micro` or `git config --global core.editor micro`.
+#
+# NOTE: upstream moved zyedidia/micro -> micro-editor/micro. The old org URL
+# still 302s, but its /releases/latest redirect lands on ANOTHER /latest URL
+# (the org rename), so the tag-parsing idiom below would resolve "latest"
+# instead of a version. Use the canonical micro-editor/micro URL.
+# Arch asset naming differs from the others: amd64 -> linux64, arm64 ->
+# linux-arm64. The tarball extracts to micro-<version>/micro.
+ARG MICRO_VERSION=latest
+RUN ARCH=$(case "${TARGETARCH}" in amd64) echo "linux64" ;; arm64) echo "linux-arm64" ;; *) echo "linux64" ;; esac) && \
+    V="${MICRO_VERSION}" && \
+    if [ "$V" = "latest" ]; then \
+      V=$(curl -sI --retry 5 --retry-delay 5 --retry-all-errors "https://github.com/micro-editor/micro/releases/latest" | awk 'tolower($1)=="location:" { sub(/\r$/,"",$2); n=split($2,a,"/"); print a[n] }'); \
+    fi && \
+    V="${V#v}" && [ -n "$V" ] && \
+    echo "Installing micro ${V}" && \
+    curl -fsSL --retry 5 --retry-delay 5 --retry-all-errors "https://github.com/micro-editor/micro/releases/download/v${V}/micro-${V}-${ARCH}.tar.gz" | tar -xz -C /tmp && \
+    install /tmp/micro-${V}/micro /usr/local/bin/micro && \
+    rm -rf /tmp/micro-${V} && \
+    micro --version
+
 # bat — syntax-highlighted cat replacement
 ARG BAT_VERSION=latest
 RUN ARCH=$(case "${TARGETARCH}" in amd64) echo "x86_64" ;; arm64) echo "aarch64" ;; *) echo "x86_64" ;; esac) && \