fix(org-roam): guard move-branch-to-roam against data loss

cj/move-org-branch-to-roam cut the subtree from the source buffer before writing the new roam file, so a failure in the demote/format/write/db-sync steps left the subtree gone from the source and not persisted anywhere — a destructive operation with no rollback.

Reordered so the node file is written and verified on disk before org-cut-subtree runs; a failed write now aborts with the source untouched. Added a no-clobber guard (refuse an existing target file) and a confirmation prompt for large subtrees (>= cj/move-org-branch-confirm-lines, 30) or buffers with unsaved changes. The source buffer is deliberately left modified and undoable rather than auto-saved, so the move stays reversible. New test drives the write-failure-preserves-source invariant via an unwritable roam dir; the existing creates-roam-file test gained the confirm mock.

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