Let plugins add contextual workspace tools (#3465)

* feat(plugins): add workspace panels and command center contributions

Give local plugins contextual UI entry points without making the daemon interpret client placement or callbacks. Panels receive stable IDs and read cached client state through required selectors with shallow equality, avoiding unrelated guest rerenders and context-discovery RPCs.

* fix(plugins): align shared client state ownership

Workspace panels and Command Center callbacks consume the same frozen client snapshots, so their adapter belongs to the plugin boundary rather than either contribution surface. Refresh the Nix dependency hash for the selector-store dependency added by this feature.

skills/paseo-plugin/SKILL.md

@@ -54,12 +54,52 @@ import type { PluginContext } from "@paseo/plugin";
 
 export default function contribute(plugin: PluginContext) {
   // Register contributions here.
-  return () => undefined;
+  return () => {};
 }
 ```
 
 Cleanup can be async. Use it for timers, watchers, sockets, and other resources created by plugin code. Paseo removes registrations, unmounts surfaces, rejects pending RPCs, closes the plugin session, and stops the subprocess when the plugin stops.
 
+## Add a workspace panel
+
+Workspace panels live beside agents, terminals, files, and diffs. Their props contain stable IDs;
+required-selector hooks read cached client state without subscribing to unrelated fields:
+
+```tsx
+import { type PluginContext, type PluginWorkspacePanelProps, useWorkspace } from "@paseo/plugin";
+import { Text } from "react-native";
+
+function Overview({ workspaceId }: PluginWorkspacePanelProps) {
+  const name = useWorkspace(workspaceId, (workspace) => workspace.name);
+  return <Text>{name}</Text>;
+}
+
+export default function contribute(plugin: PluginContext) {
+  plugin.addWorkspacePanel({
+    id: "overview",
+    title: "Workspace overview",
+    icon: "PanelsTopLeft",
+    context: "workspace",
+    Component: Overview,
+  });
+  plugin.addCommandCenterItem({
+    id: "open-overview",
+    title: "Open workspace overview",
+    icon: "PanelsTopLeft",
+    context: "workspace",
+    onSelect({ openPanel }) {
+      openPanel("overview");
+    },
+  });
+  return () => {};
+}
+```
+
+Use `useWorkspace(id, selector)` and `useAgent(id, selector)`. Selectors are required
+and their results use shallow equality. Never select the whole snapshot or add an RPC to discover
+the active workspace or agent. Command callbacks receive the selected host's `paseo`, typed
+`rpc(contract, input)`, `openSurface(id)`, and contextual `openPanel(id)` capabilities.
+
 ## Add a sidebar surface
 
 Plugin surfaces use React Native primitives and work across desktop, browser, iOS, and Android. Register the surface before its sidebar item:
@@ -93,7 +133,7 @@ export default function contribute(plugin: PluginContext) {
     icon: "ListPlus",
     surface: "main",
   });
-  return () => undefined;
+  return () => {};
 }
 ```
 
@@ -164,7 +204,7 @@ export default function contribute(plugin: PluginContext) {
     return { message: `${name}: plugins are ${config.pluginsEnabled ? "on" : "off"}` };
   });
   plugin.addSurface("main", Greeting);
-  return () => undefined;
+  return () => {};
 }
 ```
 
@@ -228,7 +268,7 @@ const issues = defineAttachmentSource({
 export default function contribute(plugin: PluginContext) {
   plugin.handle(searchIssues, ({ query }) => searchAcmeIssues(query));
   plugin.addAttachmentSource(issues);
-  return () => undefined;
+  return () => {};
 }
 ```