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.

docs/plugins.md

@@ -1,8 +1,9 @@
 # Local plugins
 
-Local plugins contribute daemon RPCs, native app surfaces, and composer attachment sources from one
-`index.tsx`. Paseo executes the server contribution in a subprocess and evaluates the client
-contribution in the app runtime. Plugin code is trusted code; this first slice does not sandbox it.
+Local plugins contribute daemon RPCs, native app surfaces, workspace panels, Command Center items,
+and composer attachment sources from one `index.tsx`. Paseo executes the server contribution in a
+subprocess and evaluates the client contribution in the app runtime. Plugin code is trusted code;
+this first slice does not sandbox it.
 
 ## Install a directory source
 
@@ -136,7 +137,7 @@ export default function contribute(plugin: PluginContext) {
     icon: "MessageCircle",
     surface: "main",
   });
-  return () => undefined;
+  return () => {};
 }
 ```
 
@@ -170,6 +171,21 @@ When the same plugin contribution exists on multiple hosts, Paseo shows it once
 adds a host picker to the screen header. The selected host supplies the bundle, RPC transport, and
 query cache. Plugin code cannot address another host.
 
+Workspace panels and Command Center items remain client contributions. The daemon transports their
+compiled bundle without interpreting placement or callbacks. Panel props contain workspace and agent
+IDs. Required-selector hooks read normalized client state synchronously and use shallow equality, so a
+panel does not subscribe to fields it does not render. Command callbacks materialize their snapshots
+only when invoked. Contribution discovery and panel opening never fetch active context through plugin
+RPC. Snapshot DTOs are deeply readonly and frozen at runtime so plugin code cannot mutate normalized
+app state or a memoized selection. Panels use one persisted
+`plugin` workspace-tab target, so reload, disable, removal, and restoration resolve through the
+current installed-plugin catalog. A missing contribution renders unavailable inside the tab.
+
+Command Center callbacks use the selected host's existing `PaseoApi` for normal Paseo operations.
+They use typed plugin RPC only for plugin-specific backend work. Navigation is limited to the
+plugin's registered global surfaces and workspace panels; plugins do not receive Expo Router or
+workspace-layout store access.
+
 ## Contribute composer attachments
 
 Register a declarative attachment source backed by a plugin RPC. Paseo owns the attachment menu,
@@ -207,7 +223,7 @@ const issues = defineAttachmentSource({
 export default function contribute(plugin: PluginContext) {
   plugin.handle(searchIssues, ({ query }) => searchAcmeIssues(query));
   plugin.addAttachmentSource(issues);
-  return () => undefined;
+  return () => {};
 }
 ```