- Add _repair_tree() post-build pass that fixes structural issues caused by
placed-set race conditions: demotes decisions with <2 children to actions,
hoists orphaned children as siblings, converts dead-end actions to solutions
- Add validation gate at commit endpoint — rejects trees that fail validation
with a 422 and descriptive error messages instead of saving invalid trees
- Update test fixtures to create valid multi-node tree structures
- Frontend: initialize currentNodeId from tree's actual root ID instead of
hardcoded 'root', fixing "Invalid tree structure" for KB-generated trees
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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