Moves all preference-resolution logic into a pure `resolveAgentForm(state, action)` reducer
with a discriminated-union action type. The hook becomes a thin React container: it holds state
via `useReducer`, dispatches actions, and owns the hydration-ref timing logic.
Removes the `__private__` export and the vi.mock-heavy live test in favour of direct pure-function
unit tests (53 tests, zero vi.mock of internal modules).
