README says that make install needs nix (closes #200)

`make install` runs `make build-bin`, which runs bun through
`nix-shell -p bun`, so on a machine without nix it stops with
`nix-shell: not found`, and the README named no such prerequisite.
Getting Started now says what its commands need and that `make install`
also needs nix, with a link to its download page. "Running the backup
from cron" says the same where it introduces `make install`, and that a
bun already installed does not help. The Makefile is unchanged, as the
plan on the issue decided. TODO.md's Next Step names the four bugs that
are open instead of saying none is.

Judgement call: "node 22 or later" is the `engines` entry in package.json.

Model: opus-5-5

TODO.md

@@ -17,7 +17,10 @@ pre-1.0
 
 # Next Step
 
-None: no implementation work is open. The check of quak against a live Ente
+Four bugs are open: https://git.eeqj.de/sneak/quak/issues/196,
+https://git.eeqj.de/sneak/quak/issues/197,
+https://git.eeqj.de/sneak/quak/issues/198 and
+https://git.eeqj.de/sneak/quak/issues/199. The check of quak against a live Ente
 account, https://git.eeqj.de/sneak/quak/issues/187, needs an account only sneak
 has, so it waits on him.
 
@@ -26,6 +29,10 @@ declares one.
 
 # Completed Steps
 
+- 2026-10-15: The README says that `make install` needs nix (issue 200), in
+  Getting Started and in "Running the backup from cron": `make build-bin` runs
+  bun through `nix-shell -p bun`. The `Makefile` is unchanged.
+
 - 2026-10-14: `lib.backup()`, and so `quak backup`, works on up to the content
   pool's limit of originals at once, 5 by default, instead of one at a time
   (issue 182). That many workers take files from one shared list, each doing all