Clean manifest plugin state at build start

Editor assessment

`fix: clean manifest plugin state at build start` states the object and the timing of the reset, which fully determines the change for a reader; a build or manifest scope would have been a natural addition. Fix correctly frames stale-state leakage as the bug. No body records the symptom users saw — duplicated or stale manifest entries — which is the one missing piece, but the subject stands alone for a change this small. The trailing PR reference is clean. Imperative, period-free, compact. Minor scope and symptom gaps; otherwise publishable.