Add RecentSessions reader over the audit chain

Exposes the last n session summaries (oldest-first) for the reviewer to
read as recent-history context. Missing/empty chain or n<=0 yields nil.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/store/audit.go

@@ -74,6 +74,25 @@ func readSummaries(path string) ([]SessionSummary, error) {
 	return out, sc.Err()
 }
 
+// RecentSessions returns up to n most-recent summaries from the audit chain in
+// oldest-first order. A missing/empty chain or n <= 0 yields (nil, nil).
+func RecentSessions(path string, n int) ([]SessionSummary, error) {
+	if n <= 0 {
+		return nil, nil
+	}
+	all, err := readSummaries(path)
+	if err != nil {
+		return nil, err
+	}
+	if len(all) == 0 {
+		return nil, nil
+	}
+	if len(all) > n {
+		all = all[len(all)-n:]
+	}
+	return all, nil
+}
+
 // AppendSession links s onto the chain (genesis prev_hash = 64 zeros), assigns
 // the next contiguous Seq, computes its Hash, and appends one JSON line with an
 // fsync.