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

The image shipped only nvim (EDITOR=nvim), a modal vi-style editor. Not
everyone is comfortable with vi keybindings, so add both a classic and a
modern non-modal option:

- 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 (<0.5% of the ~3.2 GB image). EDITOR stays nvim; both new
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.

Base-image change, so it lands on the next base-<hash> rebuild. Updates
README tool table + EDITOR note, CHANGELOG (Unreleased/Added), and
smoke-test.sh (nano + micro presence checks).

README.md

@@ -35,7 +35,9 @@ this container so all your agents share one brain.
 
 | Tool | Purpose |
 |---|---|
-| `nvim` | Neovim text editor |
+| `nvim` | Neovim text editor (modal / vi-style) |
+| `nano` | Small non-modal editor (on-screen shortcut hints) |
+| `micro` | Modern non-modal editor (Ctrl+S/Ctrl+Q keys, mouse, syntax highlighting) |
 | `tmux` | Terminal multiplexer (configured for 0-indexed sessions) |
 | `ripgrep`, `fd` | Fast file content / filename search |
 | `fzf` | Fuzzy finder |
@@ -49,6 +51,11 @@ this container so all your agents share one brain.
 | `gosu` | Privilege de-escalation in entrypoint |
 | `htop`, `tree`, `less` | Inspection utilities |
 
+The default `$EDITOR` is `nvim`. Three editors ship so you can pick your
+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`.
+
 ### Document and image tooling
 
 - `pandoc` — universal Markdown↔HTML/Org/RST/etc. converter