feat: D24 fact intervals (--as-of) and bin/stack assistant helpers. (#37)

Store valid_from/valid_to on leafs and filter search by calendar day without
overloading D16 source staleness; stack start/start-assistant wires brain + PicoClaw.

internal/brain/rank/asof_test.go

@@ -0,0 +1,34 @@
+package rank
+
+import "testing"
+
+func TestFilterAsOfKeepsXDropsY(t *testing.T) {
+	hits := []Hit{
+		{ID: "x", Text: "Andrey works at X", ValidFrom: "2024-03-01", ValidTo: "2025-07-15"},
+		{ID: "y", Text: "Andrey works at Y", ValidFrom: "2025-07-16", ValidTo: ""},
+		{ID: "legacy", Text: "always true claim", ValidFrom: "", ValidTo: ""},
+	}
+	out := FilterAsOf(hits, "2025-01-01")
+	if len(out) != 2 {
+		t.Fatalf("len=%d want 2: %+v", len(out), out)
+	}
+	if out[0].ID != "x" || out[1].ID != "legacy" {
+		t.Fatalf("got %+v", out)
+	}
+	if FilterAsOf(hits, "") == nil || len(FilterAsOf(hits, "")) != 3 {
+		t.Fatal("empty as-of must keep all")
+	}
+}
+
+func TestParseArgsAsOf(t *testing.T) {
+	opt, err := ParseArgs([]string{"who works where", "--as-of", "2025-01-01", "--json"})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if opt.AsOf != "2025-01-01" {
+		t.Fatalf("AsOf=%q", opt.AsOf)
+	}
+	if _, err := ParseArgs([]string{"q", "--as-of", "not-a-date"}); err == nil {
+		t.Fatal("expected bad as-of error")
+	}
+}