chore: cap every CI job with timeout-minutes (closes #294)

No workflow set timeout-minutes, so a hung build or browser held the
shared runner until the server's own limit, hours later. check now stops
at 10 minutes, e2e-firefox at 15 and e2e-chrome at 20: each is over two
and a half times the job's slowest cold-cache run. README "In CI" records
the measured times and the caps.

Model: opus-5-5

README.md

@@ -643,12 +643,20 @@ Nothing in either job can pass vacuously. There is no `continue-on-error` and no
 build fails, and when the browser fails to start; the Chrome harness aborts the
 suite outright if its network interception is not in effect.
 
-Measured on this repo's runner: `e2e-chrome` about 1m55s cold, almost all of it
-the one-time pull of the pinned ~800MB Playwright layer, and well under a minute
-once that layer is cached. `e2e-firefox` about 1m05s cold, and it caches its
-Firefox and geckodriver downloads the same way. The `e2e-chrome` figures predate
-the two cases that wait for a receipt to end in error, which add about two
-minutes of real waiting.
+Measured on this repo's runner in the green runs of early October 2026, from a
+warm docker cache to a cold one: `check` 49s to 3m37s, `e2e-chrome` 1m44s to
+4m48s, and `e2e-firefox` 31s to 4m07s. A cold cache adds three to four minutes
+to each job, spent rebuilding its image: reinstalling dependencies and, for
+`e2e-firefox`, installing Firefox, geckodriver and their system libraries. Those
+`e2e-chrome` runs predate the cases that wait in real time for a receipt to end
+in error. `make test-e2e` now takes 3m51s locally with its image cached, so a
+cold `e2e-chrome` run comes to about seven minutes.
+
+Every job has a `timeout-minutes` cap, so a hung build or browser ends the job
+instead of holding the shared runner: `check` 10 minutes, `e2e-firefox` 15 and
+`e2e-chrome` 20, each over two and a half times the job's slowest cold run. A
+job that reaches its cap has hung; read it as a hang, not as a slow run to
+retry.
 
 ### Element id guard (part of `make check`)