refactor(frontend): add @RATIONALE/@REJECTED to 5 C:4 contracts
- ConfigTabForm: rationale for single-tab vs wizard; rejected multi-step
- ValidationTaskForm: rationale for multi-step wizard; rejected flat form,
tabs, dynamic schema generation
- GitWorkspacePanel: rationale for IntersectionObserver lazy diff chunking;
rejected virtual scroll, server pagination, Web Worker
- Git.ManagerModel: rationale for model-first (cross-operation invariants,
L1 testability); existing @REJECTED preserved
- AgentChat.Component: rationale for reusable component vs inline;
rejected Web Component, iframe