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>

agent/multi_file_diff_review.go

@@ -79,7 +79,7 @@ func MultiFileDiffReview(c *ui.Context, decisions *[]FileDecision, files []FileC
 					slog.Debug("mujica: multi_file_diff_review select", "path", files[i].Path)
 				}
 			}
-			var list *ui.Element
+			var list ui.Element
 			if len(files) > agentVirtualRows {
 				list = ui.List(c, nil, len(files), row)
 			} else {
@@ -109,7 +109,7 @@ func MultiFileDiffReview(c *ui.Context, decisions *[]FileDecision, files []FileC
 func multiFileRow(c *ui.Context, f FileChange, d FileDecision, selected bool) bool {
 	k := core.Tokens(c)
 	u := core.Density(c).Unit()
-	row := ui.ButtonBase(c).Key(f.Path).Justify(ui.Start).Gap(u*2).MinHeight(core.Density(c).ControlHeight()).
+	row := ui.ButtonBase(c.Key(f.Path)).Justify(ui.Start).Gap(u*2).MinHeight(core.Density(c).ControlHeight()).
 		Padding(0, u*2).Radius(theme.ControlRadius).Label(f.Path)
 	internal.Ring(row, k.Focus, theme.ControlRadius)
 	dc := agentDiffOf(row, f)
@@ -133,7 +133,7 @@ func multiFileRow(c *ui.Context, f FileChange, d FileDecision, selected bool) bo
 func fileChangeDecide(c *ui.Context, d *FileDecision, f FileChange, locked bool) bool {
 	u := core.Density(c).Unit()
 	changed := false
-	holder := ui.Column(c).Key(f.Path).Grow(1).Shrink(1).Gap(u * 2)
+	holder := ui.Column(c.Key(f.Path)).Grow(1).Shrink(1).Gap(u * 2)
 	dc := agentDiffOf(holder, f)
 	holder.Children(func() {
 		ui.Row(c).Gap(u * 2).AlignItems(ui.Center).Children(func() {