Simplify MCP App UIs for basic branch

Remove advanced features to be kept in mcp-ui-apps-advanced:
- Strip labels, assignees, milestones, issue types, repo picker from issue-write
- Strip repo picker, branch selectors from pr-write
- Delete ui_get tool (ui_tools.go, ui_tools_test.go, ui_get.snap)
- Remove UIGet registration from tools.go

Basic forms retain: title, body, submit with _ui_submitted,
draft/regular split button (PR), MarkdownEditor, and SuccessView.

Author: mattdholloway

pkg/github/__toolsnaps__/ui_get.snap

@@ -295,9 +295,6 @@ func AllTools(t translations.TranslationHelperFunc) []inventory.ServerTool {
 		GetLabelForLabelsToolset(t),
 		ListLabels(t),
 		LabelWrite(t),
-
-		// UI tools (insiders only)
-		UIGet(t),
 	}
 }