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, runs it by that tag,
and removes it in the scripts' existing exit and interrupt cleanup, so no
run leaves an image behind. The image-ID file that guarded against a
moving tag is no longer needed. README.md's End-to-End Tests section says
so.

Model: opus-5-5

README.md

@@ -347,6 +347,12 @@ are outside `make check`. Neither opens the popup from the toolbar button: both
 load its page in an ordinary tab, so what the toolbar popup itself adds, its
 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, and removes that
+image when it exits, an interrupt included. Two clones can therefore run the
+same suite at once without one build taking the other's image away, and no run
+leaves an image behind.
+
 ### Chrome (`make test-e2e`)
 
 `make test-e2e` builds `dist/chrome/` and drives the **real popup in a real