buildOutputValidator already exists in task/executor.ts but only ran on
the fallback JSON parse path. Subagents that called yield directly with
a data payload skipped validation entirely, so a schema-conforming-
empty-object (e.g. `{}` against a schema requiring `findings`) was
returned as a successful task.
finalizeSubprocessOutput now invokes the validator on every yield path
and on the fallback completion path. On failure the result carries
error="schema_violation", a typed message, the missing required field
list, and a truncated preview of the offending data. exitCode=1 and
isError=true so existing consumers in task/index.ts surface it as a
failed task without code changes.
