feat(config): add TAPIR_ONBOARD_SUMMARIZE_COUNT (default 3, hard cap 5)

Bounds the connect-time onboarding summary burst (Feature 1). Hard-capped at 5
and clamped (negative->0, >cap->cap) so onboarding can never bulk-fetch; 0
disables. The cap bounds COUNT only — every fetch still flows through the shared
caption rate gate (ADR-014).

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

internal/config/config_test.go

@@ -53,6 +53,38 @@ func TestLoad_AppliesDefaults(t *testing.T) {
 	}
 }
 
+func TestLoad_OnboardSummarizeCount(t *testing.T) {
+	cases := []struct {
+		name, env string
+		want      int
+	}{
+		{"default", "", defaultOnboardSummarizeCount},
+		{"explicit", "4", 4},
+		{"zero disables", "0", 0},
+		{"clamped to hard cap", "50", maxOnboardSummarizeCount},
+		{"negative clamps to zero", "-3", 0},
+	}
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			setEnv(t, map[string]string{"TAPIR_ONBOARD_SUMMARIZE_COUNT": c.env})
+			cfg, err := Load()
+			if err != nil {
+				t.Fatalf("Load: %v", err)
+			}
+			if cfg.OnboardSummarizeCount != c.want {
+				t.Fatalf("OnboardSummarizeCount = %d, want %d", cfg.OnboardSummarizeCount, c.want)
+			}
+		})
+	}
+}
+
+func TestLoad_OnboardSummarizeCountInvalid(t *testing.T) {
+	setEnv(t, map[string]string{"TAPIR_ONBOARD_SUMMARIZE_COUNT": "three"})
+	if _, err := Load(); err == nil {
+		t.Fatal("Load: want error for non-numeric TAPIR_ONBOARD_SUMMARIZE_COUNT")
+	}
+}
+
 func TestLoad_ParsesValues(t *testing.T) {
 	setEnv(t, map[string]string{
 		"TAPIR_USER_ID":               "11111111-1111-1111-1111-111111111111",