fix(agent): fix race in TestMaybeSummarize_LogsTriggerDecisionOverThreshold (#1347)

maybeSummarize spawns a background goroutine that logs via slog.Info
before calling the LLM provider -- correct, intentional async design,
not a bug. The test read the shared log buffer (buf.String(), inside
captureSlog) immediately after maybeSummarize returned, i.e. right
after the goroutine was merely spawned, not after it finished logging
-- an unsynchronized concurrent read/write on bytes.Buffer, caught by
go test -race. This was surfaced by an unrelated PR's CI run
(nextlevelbuilder/goclaw#1346), confirmed unrelated to that PR's
actual changes.

Moved the existing done-channel wait inside captureSlog's closure, so
the buffer read only happens after the background goroutine has
signalled it reached provider.Chat -- which happens strictly after
the slog.Info write on this path. Proper happens-before via the
channel, no sleep/poll, no production code changed.

Verified 10/10 passes under go test -race -count=10, plus full
package regression run clean.

Co-authored-by: Bruno Clermont <bruno.clermont@gmail.com>

internal/agent/loop_history_sanitize_max_tokens_test.go

@@ -234,13 +234,19 @@ func TestMaybeSummarize_LogsTriggerDecisionOverThreshold(t *testing.T) {
 
 	logs := captureSlog(t, func() {
 		loop.maybeSummarize(context.Background(), "test-session-key")
-	})
 
-	select {
-	case <-done:
-	case <-time.After(5 * time.Second):
-		t.Fatal("timed out waiting for maybeSummarize to call provider.Chat")
-	}
+		// Wait for the background summarize goroutine to reach the point where it
+		// calls provider.Chat (which happens strictly after the "compact_budget"
+		// slog.Info write). This must happen INSIDE the captureSlog closure so the
+		// buffer is not read until the background goroutine's logging is done —
+		// otherwise buf.String() races with the goroutine's concurrent slog.Info
+		// write to the same bytes.Buffer under -race.
+		select {
+		case <-done:
+		case <-time.After(5 * time.Second):
+			t.Fatal("timed out waiting for maybeSummarize to call provider.Chat")
+		}
+	})
 
 	assertLogContains(t, logs,
 		"compaction_decision",