preview_merge could only see file-overlap cleanliness via git merge-tree, never
whether the result compiles -- the two costliest findings of the 2026-08-06 batch
run were both merge-tree-clean but build-broken. When a list has a verify command
configured, a clean preview is now additionally materialized (via a commit-tree +
detached scratch worktree, outside the real repo, always cleaned up) and
built/tested there, without ever touching the real working tree.
preview_merge always attempts a verify run when a command is configured;
preview_merge_set only does when its new runVerify parameter is set (default off),
so a set preview never starts N builds unasked. The post-merge verify gate is
unchanged.
mika kuns