fix: each browser-suite run builds and removes an image of its own (closes #547)

script/test-e2e and script/test-e2e-firefox tagged their image with one
fixed name. When two clones ran a suite at once, the second build moved
that tag, the image store deleted the first run's now-untagged image, and
that suite exited 125 without running a test. Each run now tags its image
with the suite's name and twelve random hex digits and runs it by that
tag. The cleanup that runs on exit, Ctrl-C, kill and a closed terminal
removes the run's container if docker's client left it running, then the
image. The image-ID file that guarded against a moving tag is gone.
README.md's End-to-End Tests section says which endings clean up.

Model: opus-5-5

README.md

@@ -350,8 +350,8 @@ size and its closing when it loses focus, is covered by neither.
 Each run builds its image under a tag of its own, `autistmask-e2e-chrome:` or
 `autistmask-e2e-firefox:` followed by twelve random hex digits, so two clones
 can run the same suite at once without one build taking the other's image away.
-The run removes that image, and its container if one is still running, after
-the suite passes or fails, on Ctrl-C, on `kill`, and when the terminal or SSH
+The run removes that image, and its container if one is still running, after the
+suite passes or fails, on Ctrl-C, on `kill`, and when the terminal or SSH
 session it runs in closes. A run killed outright (`kill -9`) cannot clean up:
 its image stays under its tag until removed by hand with `docker image rm`.