The duplication gate flagged three ACP/SDK clones; the shared halves move
into dsh-subagent-subprocess as provider.ts: NO_START_CAPABILITIES (frozen
all-false advertisement), assertPositiveFinite (prefix-parameterized timing
validation), settleRunResult (never-reject result settlement with contained
onError sink and listener hygiene), and subprocessRunHandle (idempotent
dispose publication). Both backends now compose these; the previously
unreachable cancelled-rejection branch is directly unit-tested at the
library level instead of v8-ignored in each backend. knip learns the
subagent-sdk workspace (e2e entry outside vitest unit includes).