fix(repl): the bring-up build's prompt waits for a line instead of answering itself without end

Built with EMERGENCY_CONSOLE_ENABLED=1 and booted with no identity logged
in, the prompt printed "ok>  ok" without end, 1.4 million times in 200
seconds (logs/20261013-210442).  sk_console_readline() gave up at once
whenever no identity was present.  The production build answers that by
going silent until a login; the bring-up build, which needs no login,
took it as an empty line and asked again.  The give-up is now the
production build's only.

Found rehearsing boards/BRINGUP.md; it predates the console change of
c9519173 (the image built from the commit before that did the same).

The riscv64 bring-up build, no thumbdrive, three lines typed
(logs/20261013-235129): one prompt that waits, and 3, 56 and 81.
Acceptance of the production build, one QEMU at a time: amd64
(-235255), aarch64 (-235411), riscv64 (-235620) to the zuse prompt, no
unknown word, hash 0x08873e0f44b7cb2a on all three.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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