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_test.go

@@ -75,3 +75,45 @@ func TestTamperDetected(t *testing.T) {
 		t.Fatalf("tamper on line 2 should be reported, got %v", err)
 	}
 }
+
+func TestRecentSessionsReturnsLastNOldestFirst(t *testing.T) {
+	path := auditFixture(t)
+	for _, id := range []string{"a", "b", "c"} {
+		if err := AppendSession(path, sampleSummary("session-"+id)); err != nil {
+			t.Fatalf("append %s: %v", id, err)
+		}
+	}
+	got, err := RecentSessions(path, 2)
+	if err != nil {
+		t.Fatalf("recent: %v", err)
+	}
+	if len(got) != 2 {
+		t.Fatalf("want 2 summaries, got %d", len(got))
+	}
+	if got[0].SessionID != "session-b" || got[1].SessionID != "session-c" {
+		t.Fatalf("want [b c] oldest-first, got [%s %s]", got[0].SessionID, got[1].SessionID)
+	}
+}
+
+func TestRecentSessionsFewerThanN(t *testing.T) {
+	path := auditFixture(t)
+	_ = AppendSession(path, sampleSummary("session-a"))
+	got, err := RecentSessions(path, 5)
+	if err != nil {
+		t.Fatalf("recent: %v", err)
+	}
+	if len(got) != 1 || got[0].SessionID != "session-a" {
+		t.Fatalf("want [a], got %+v", got)
+	}
+}
+
+func TestRecentSessionsMissingOrZero(t *testing.T) {
+	if got, err := RecentSessions(auditFixture(t), 5); err != nil || got != nil {
+		t.Fatalf("missing chain: want (nil,nil), got (%+v,%v)", got, err)
+	}
+	path := auditFixture(t)
+	_ = AppendSession(path, sampleSummary("session-a"))
+	if got, err := RecentSessions(path, 0); err != nil || got != nil {
+		t.Fatalf("n=0: want (nil,nil), got (%+v,%v)", got, err)
+	}
+}