feat(app-boot): register cordis:group beside cordis:include

A composition that shares one `isolate` realm across rows needs a
`cordis:group` row, and a preset living outside this workspace — the
authored ones under the Harness home — cannot resolve
`@cordisjs/plugin-group` by name: Node's upward `node_modules` walk
never reaches the harness from there. Registering it as a loader builtin
beside `cordis:include` loads both through the ambient module pipeline
instead.

Record it in the preset Agent Note, which leans on the realm vocabulary
without saying where the group row comes from, and drop the preset
README's limitation claiming this builtin is unavailable — it described
the state this change ends.

The test's assertion had a vacuous escape: `provide` mints the root
symbol unconditionally, so the `rootKey === undefined` disjunct could
never hold and the comment claiming the root realm never learned the
name was wrong. Pin both halves — the symbol exists, nothing is stored
under it — and clean up the global the fixture writes.

packages/ui/app-boot/src/index.ts

@@ -13,6 +13,7 @@ import * as yaml from 'js-yaml'
 import { Context, type FiberState } from 'cordis'
 import Loader, { type Entry, type EntryOptions } from '@cordisjs/plugin-loader'
 import Include, { applyEntryPatches, entryListSchema, type PatchOptions } from '@cordisjs/plugin-include'
+import Group from '@cordisjs/plugin-group'
 import { dshHomePath } from '@deepseek-ai/dsh-paths'
 import type {} from '@cordisjs/plugin-hmr'
 // Side-effect type import: resolves `ctx.get('systemPrompt')` to the service.
@@ -376,6 +377,12 @@ export async function mountRootInclude(
   patches: readonly PatchOptions[] = [],
 ): Promise<Entry | undefined> {
   ctx.loader.builtins.include = Include
+  // `cordis:group` alongside it: a group row is how a composition gives one
+  // `isolate` realm to a provider and its consumers together, and an agent
+  // preset living outside this workspace cannot resolve `@cordisjs/plugin-group`
+  // by name. Both builtins load through the ambient module pipeline, so neither
+  // depends on the included tree's own specifier resolution.
+  ctx.loader.builtins.group = Group
   // Pinned id: the bootstrap include is app glue, not a config row, and its
   // id appears in Loader failure chains — a random id would make startup
   // diagnostics unstable across runs (and snapshot fixtures).