Retain the agent's final report on the benchmark result record for read
tasks, so a failed read is diagnosable directly from the stored record
instead of only carrying an opaque `incorrect` tag. The runner resolves
the scoring spec once and records `run.finalReport` when the spec is a
read; mutation records omit the field entirely. The sample store needs no
change — it serializes whatever record it is handed.
This is the prerequisite for confirming the read-open-issue-count failure
from real report text before the state-aware count-line change (task 0033).
