resolveTakesSourceId() caught every error from resolveSourceId() and fell
back to undefined, which restores the pre-#2698 unscoped (cross-source)
slug lookup for takes add/update/supersede/resolve. resolveSourceId()
only ever throws when a source was explicitly in play (an invalid or
unregistered GBRAIN_SOURCE, a .gbrain-source dotfile pointing at a
source that doesn't exist, or a genuine DB error) — it never throws for
"nothing configured," which resolves cleanly to the seeded 'default'
source. So swallowing the error had no legitimate case to protect and
only reintroduced the cross-source write bug on any resolution failure.
Let it propagate so the write is blocked instead.
Adds regression coverage for both the unchanged happy path (no source
configured resolves cleanly) and the newly fail-closed path (an
unregistered GBRAIN_SOURCE blocks the write instead of falling back to
an unscoped lookup).
