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.

scripts/smoke-test.sh

@@ -3,6 +3,7 @@
 #
 # Verifies:
 #   - Core binaries are on PATH and runnable
+#   - non-modal editors nano + micro are present (alongside nvim)
 #   - opencode itself starts and prints a version
 #   - Entrypoint runs cleanly as non-root after UID adjustment
 #   - Generated opencode.json has the expected shape
@@ -118,6 +119,8 @@ run "node"              "node --version"
 run "npm"               "npm --version"
 run "git"               "git --version"
 run "nvim"              "nvim --version | head -1"
+run "nano"              "nano --version | head -1"
+run "micro"             "micro --version"
 run "bat"               "bat --version"
 run "eza"               "eza --version | head -1"
 run "zoxide"            "zoxide --version"