Garden: suppress the baseline explanation in the unrunnable check report #186

Follow-ups from #178 (merged as 8a89f2d), both cosmetic and both in the new "could not run the checks" path.

1. Misleading baseline sentence (cubic P3, feat(flows): use the 180-minute cap for Software Garden robustness, not a longer plan #178 (comment)). The unrunnable report still emits the generic base-commit warning, even though that path deliberately skips the base comparison and leaves baseline empty. A reader is told a comparison was attempted and came back unknown, when none was tried. Suppress the baseline explanation for unrunnable, or give it an explicit baseline case.

2. Close the verdict set. unrunnable is a new verdict string reaching FLOW_CHECK_REPORT_COMMAND, and the generated flow's verdictOf guards only the baseline, not check. The value is produced by our own command so it cannot be injected, but keeping the flow's verdict set closed to a known list is tidier and would catch a future typo.

Neither affects a hosted run: Cloud's image always has coreutils timeout, so only a local kit run can reach unrunnable at all.