Run make try against a temporary directory (closes #12)

`make try` ran against a fixed directory on one person's machine. It
failed for anyone else, and where that directory existed it rewrote the
checksum attributes and modification times of real files.

It now writes three small files into a new directory from `mktemp -d`,
runs the same sequence of commands on it, and removes it with an EXIT
trap. All steps run in one shell, so the trap also removes the
directory when a step fails.

`TODO.md`: the `try` sentence leaves Next Step and is recorded under
Completed Steps.

Model: opus-5-5

TODO.md

@@ -20,12 +20,13 @@ have landed since the tag.
 Policy scaffold commit: add LICENSE, REPO_POLICIES.md, .editorconfig,
 .golangci.yml, and a comprehensive .gitignore (currently only the
 attrsum binary), and extend the Makefile (only test/build/clean/try
-today) with lint, fmt, fmt-check, check, and hooks targets. Fix the try
-target to use a temp fixture instead of the hardcoded
-$(HOME)/Documents/_SYSADMIN/cyberdyne path.
+today) with lint, fmt, fmt-check, check, and hooks targets.
 
 # Completed Steps
 
+* 2026-10-05: `make try` runs on three small files in a temporary
+    directory that it removes afterwards, also when a step fails,
+    instead of on a fixed directory on one person's machine
 * 2026-10-02: `attrsum --version` reports the git tag or short commit,
     stamped by `make build` and by a plain `docker build .` of a clone;
     `.dockerignore` sends `.git` without `.git/config` and keeps a