feat: persist agent report on read result records (task 0032)

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).

bench/result.ts

@@ -72,6 +72,15 @@ export interface ResultRecord {
 
   /** Pass/fail outcome with a failure tag. */
   outcome: Outcome;
+
+  /**
+   * The agent's final report, retained for read tasks so a failed read is
+   * diagnosable directly from the record — the exact text the agent submitted,
+   * distinguishing a wrong answer from a right one phrased in words the checker's
+   * accepted-phrasing list did not match. Absent for mutation tasks, which are
+   * scored by diffing repository state and have no agent report to record.
+   */
+  report?: string;
 }
 
 /**