test: widen Kiro detached-child promptness margin for loaded CI runners

[steipete]

Summary

• fetch returns promptly when usage helper spawns a detached child asserted elapsed < 8, which flaked on loaded GitHub macOS runners (12.6s observed on run 30391595404) even though the probe behaved correctly.
• The detached child now sleeps 60s (was 30s) and the ceiling is raised to 20s with a comment, so the assertion still fails hard if the probe ever waits for the child, while tolerating slow shared runners.

Testing

• swift test --filter KiroStatusProbeTests (55 tests pass)
• make check (0 violations)