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loading and error states, keeps previous results, and drops superseded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.

docs/plugins.md

@@ -691,6 +691,37 @@ The overflow sheet's content portals to the app root, away from the composer. Pa
 `useComposerSheetBridge()` as the sheet's `contextBridge`, or `useComposer()` returns nothing and
 the sheet draws no plugin rows.
 
+## Contribute New workspace items
+
+`addNewWorkspaceItem` mounts a plugin component on the New workspace screen. The public contract
+is the [New workspace items reference](../public-docs/plugins/reference.md#new-workspace-items).
+
+The screen's choices that a plugin may change live in one plain store,
+`createNewWorkspaceSetup` (`packages/app/src/screens/new-workspace/setup-model.ts`): the base
+picker selection, the manual isolation, the branch name and the workspace metadata. The pickers and
+the plugin commands write to it, so a command sees what an earlier command left. Project and host
+stay in the screen's hooks (`useNewWorkspaceHostSelector` reconciles them against each other), and
+they are read-only to plugins; a command that switched them would unmount the item that called it.
+
+The screen reports what it derives from those hooks (the lock, the project, whether the project
+can make worktrees, whether the host supports metadata, and the pickers' own handlers) to the store
+with `setup.report` on every render. Commands read it when they run, never from the render that
+built them. `chooseIsolation` and `chooseBase` are the pickers' handlers, so a plugin command has the
+effects of a pick. Add a capability by extending the store and its facts; do not read screen state
+from a plugin component.
+
+`ensureWorkspace` sends the store's `branch` as `source.branchName` only for a branch-off worktree
+(`checkoutRequest.action === "branch-off"`), and its `metadata` on `workspace.create` when it is
+not empty. The server uses `branchName ?? worktreeSlug` as the new branch.
+
+Items render through `ComposerControl` with `surface="field"`: a `FieldTrigger` (a badge on
+desktop, a full-width field when compact) and the same pending state, error toast and list as the
+composer's controls. `ComposerControlLockProvider` carries the screen's lock to them. Plugin
+`Combobox` is a `select` item with `remoteSearch`: the app `Combobox` does not filter locally, and
+`PluginCombobox` (`plugins/react-native/composer-controls.tsx`) runs `search` through
+`useFetchQuery` with `dataShape: "list"`, which keeps the previous results while a newer query runs.
+`plugins/item-host.tsx` is the host shared by composer and New workspace items.
+
 ## Contribute timeline items
 
 Timeline transformers and renderers are client contributions. The daemon's canonical rows and