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/mcp_server_list.go

@@ -53,7 +53,7 @@ type MCPServerListOptions struct {
 
 // MCPServerListResult reports this frame's request.
 type MCPServerListResult struct {
-	Element *ui.Element
+	Element ui.Element
 	Action  MCPAction
 	// Server is the ID the action is for.
 	Server string
@@ -105,7 +105,7 @@ func mcpServerRow(c *ui.Context, s MCPServer, locked, still bool) MCPAction {
 	words := [...]string{"stopped", "connecting", "connected", "error"}
 	word := core.Msg(c, "mcp_server_list."+words[s.State])
 	act := MCPNone
-	ui.Row(c).Key(s.ID).Gap(u*3).AlignItems(ui.Center).MinHeight(d.ControlHeight()+u*2).Padding(u*2, u*3).
+	ui.Row(c.Key(s.ID)).Gap(u*3).AlignItems(ui.Center).MinHeight(d.ControlHeight()+u*2).Padding(u*2, u*3).
 		Label(s.Name + ", " + word).Children(func() {
 		switch s.State {
 		case MCPConnecting: