Upgrade MyGo to v0.5.5

MyGo v0.5.5 returns ui.Element as a checked value handle instead of a
pointer. Migrated with `mygo migrate-ui`, then fixed what the codemod
leaves to review:

- Element nil checks become .Valid(); absent elements are ui.Element{}.
- Input handlers and paint callbacks run outside the build pass, where
  elements expire and Context methods are inert: read IsDisabled into a
  local during the build, keep a ui.Handle for Focus, use c.Services()
  for clipboard access, create ui.Local state during the build.
- Add core.AutoFocus: v0.5.5 records an overlay's opener after its
  content builds, so same-frame autofocus lost focus restore on close.
  It focuses one frame later; cascader rows defer their focus likewise.
- core.OverlayLayer identifies build passes by the root element, not
  c.Now(), which repeats across passes and under the Tester clock.
- Context menus survive the opening press counted as PressedOutside;
  split button tooltip no longer reorders its open menu; disabled
  menubar rows pass presses to the panel; Form defers blur-started
  async checks so the growing message no longer moves Submit away
  from the release; Combobox drops its disabled copy now that MyGo
  blocks typing under a disabled wrapper (canary flipped to a guard).
- The Tester clock is virtual: tests use Advance/Now instead of
  time.Sleep/time.Now and read element state inside the view. The web
  mirror advances the clock to wall time before each frame and input.
- AGENTS.md and docs describe the value Element API and the new pin.

Snapshot goldens are darwin-only and were not re-verified here.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

chat/capability_cards_test.go

@@ -35,10 +35,10 @@ func TestCapabilityCardsExample(t *testing.T) {
 
 func TestCapabilityCardsColumnsAndDisabled(t *testing.T) {
 	got := ""
-	var g *ui.Element
+	built := false
 	tt := chatTester(900, 600, func(c *ui.Context) {
 		v := CapabilityCards(c, chatCards(), CapabilityCardsOptions{Columns: 1, Disabled: true})
-		g = v.Element
+		built = v.Element.Valid()
 		if v.Example != "" {
 			got = v.Example
 		}
@@ -51,7 +51,7 @@ func TestCapabilityCardsColumnsAndDisabled(t *testing.T) {
 	}
 	a, _ := tt.Find("Write")
 	b, _ := tt.Find("Plan")
-	if b.Y <= a.Y || g == nil {
+	if b.Y <= a.Y || !built {
 		t.Fatalf("one column should stack cards: %v %v", a, b)
 	}
 }