fix(scripts): isolate SMS readiness probes and reuse device discovery

The SMS readiness watcher hardcodes a Homebrew adb executable and queries devices twice per polling pass. Its deadline regression fixture replaces curl/ioreg on PATH but still executes the host adb. In the full app-core suite the two-second watch deadline expired before the fake curl started, so the test never exercised forced probe termination.

Acceptance: honor the existing Android installer ADB environment override in the watcher; preserve the default command; reuse one device snapshot per pass; make CLI timing fixtures control every external probe and retain proof that the TERM-ignoring child actually ran and was killed. No real device operation is needed for these timing checks. Observed at fe66ed82b7 during #31532 final qualification.