feat(plugins): turn Info panel into a PI WEB status view

Rework the bundled Info plugin from a demo into an always-available
status view: running and installed versions, installation details,
release state, and per-service health rendered from host-provided
state, plus machine and workspace details. Replace the window.alert
demo action with a Copy PI WEB Diagnostics action that copies a
plain-text summary for bug reports.

Add state.selectedMachine to the stable plugin runtime state so
actions and other runtime callbacks can read the selected machine's
identity, and split the plugin into a copy-paste-friendly skeleton
(pi-web-plugin.ts) and replaceable internals (infoInternals.ts).

src/plugin-api.ts

@@ -66,6 +66,8 @@ export interface PluginMachine {
 }
 
 export interface PluginRuntimeState {
+  /** Identity of the currently selected machine. Undefined only on older hosts or before machines load. */
+  selectedMachine?: PluginMachine;
   selectedWorkspace?: Workspace;
   selectedSession?: unknown;
   workspaceTool?: string;