sz-final-review-loop: wait with a wake armed, not blocked in sz-wait-for

The loop's agents blocked in a foreground sz-wait-for because nothing could
bring an agent back from a background one. Both harnesses now can: pi's
alarm fires when a when_done command exits, and Claude Code starts a new
turn when a run_in_background Bash task exits (checked on 2.1.310). So an
agent arms sz-wait-for as a wake and ends its turn: no context resent every
30 minutes to re-arm, the human can talk to it while it waits, and "arm,
then signal" becomes literal. The foreground wait stays for a harness with
neither.

The invariant is unchanged in substance: never idle, where idle now means
the other side's turn with no wake armed. sz-wait-for stays the watcher, for
its settle window.

pi-subagent: §5 says how to be woken in Claude Code too.

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