feat(plugins): New workspace items set the branch and workspace metadata

addNewWorkspaceItem mounts a plugin field on the New workspace screen, and
useNewWorkspace() exposes host, project, isolation, base ref, branch and
metadata with commands that follow the composer item rules: they act on the
screen's state when they run, resolve once applied, reject while the screen
is locked, and take JSON payloads. Host and project are read-only because a
command that switched them would unmount the item that called it.

The screen's pickable state (base selection, manual isolation, branch,
metadata) moves into one plain store so pickers and plugin commands write to
the same place; the screen reports derived facts to it on each render. The
branch is sent as source.branchName only for a branch-off worktree, and
metadata on workspace.create.

Items render through ComposerControl with a new "field" surface (badge on
desktop, full-width field when compact). Combobox ships here: Paseo owns the
loading and error states, keeps previous results, and drops superseded
searches.

packages/app/src/plugins/evaluate.ts

@@ -133,6 +133,7 @@ export function runPluginClientBundle(
     legacySidebarItems: [],
     filePanels: [],
     composerItems: [],
+    newWorkspaceItems: [],
     workspacePanels: [],
     commandCenterItems: [],
     clientSlashCommands: [],
@@ -149,6 +150,7 @@ export function runPluginClientBundle(
   };
   const filePanelIds = new Set<string>();
   const composerItemIds = new Set<string>();
+  const newWorkspaceItemIds = new Set<string>();
   const workspacePanelIds = new Set<string>();
   const commandCenterItemIds = new Set<string>();
   const clientSlashCommandNames = new Set<string>();
@@ -286,6 +288,20 @@ export function runPluginClientBundle(
         () => composerItemIds.delete(normalizedId),
       );
     },
+    addNewWorkspaceItem(contribution) {
+      const normalizedId = requireId(contribution.id, "New workspace item id");
+      if (newWorkspaceItemIds.has(normalizedId))
+        throw new Error(`Duplicate New workspace item: ${normalizedId}`);
+      if (!isComponentType(contribution.Component)) {
+        throw new Error(`New workspace item ${normalizedId} is not a component`);
+      }
+      newWorkspaceItemIds.add(normalizedId);
+      return register(
+        collector.newWorkspaceItems,
+        { id: normalizedId, Component: contribution.Component },
+        () => newWorkspaceItemIds.delete(normalizedId),
+      );
+    },
     addSidebarHeaderItem(contribution: PluginSidebarItemContribution) {
       return addSidebarItem("header", contribution);
     },
@@ -603,6 +619,7 @@ export function runPluginClientBundle(
     legacySidebarItems: collector.legacySidebarItems,
     filePanels: collector.filePanels,
     composerItems: collector.composerItems,
+    newWorkspaceItems: collector.newWorkspaceItems,
     workspacePanels: collector.workspacePanels as EvaluatedPlugin["workspacePanels"],
     commandCenterItems: collector.commandCenterItems,
     clientSlashCommands: collector.clientSlashCommands,