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/context_chips.go

@@ -20,7 +20,7 @@ type ContextChipsOptions struct {
 
 // RemoveResult carries the element and removed item ID.
 type RemoveResult struct {
-	Element *ui.Element
+	Element ui.Element
 	removed string
 }
 
@@ -48,7 +48,7 @@ func ContextChips(c *ui.Context, items []ContextItem, opts ContextChipsOptions)
 			shown = min(shown, limit)
 		}
 		for _, it := range items[:shown] {
-			ui.Row(c).Key(it.ID).Gap(u).AlignItems(ui.Center).Padding(0, 0, 0, u*2).MinHeight(core.Density(c).ControlHeight()).
+			ui.Row(c.Key(it.ID)).Gap(u).AlignItems(ui.Center).Padding(0, 0, 0, u*2).MinHeight(core.Density(c).ControlHeight()).
 				Radius(theme.ControlRadius).Background(k.Surface).Border(theme.BorderWidth, k.Border).Role(ui.RoleListItem).
 				Children(func() {
 					ui.Icon(c, icons.Must(it.Kind.icon())).Size(14, 14).Shrink(0).TextColor(k.AccentText)