feat(settings): serve every registered namespace and key plugin cards on it

A plugin that registered a settings namespace could not reach the browser
configuration page: the api-proxy filtered every read and gated every
write through two hardcoded namespace lists, and the plugin configuration
section rendered an unordered list of cards carrying an opaque id rather
than the namespace they edit. Both gates lived in this repository, so a
user-authored plugin was configurable only by hand-editing settings.yaml.

The proxy now serves whatever ctx.settings.describe() returns and adds no
boundary of its own; a name no registration answers folds into the seam's
own settings-rejected, and the settings-not-exposed code retires. The
settings seam is untouched: which client may read a namespace, and which
page renders it, are facts about consumers.

settings.plugin.item becomes a keyed slot whose key is the namespace a
card edits, following tool.call.toolview. The section reads describe once
and dispatches the intersection of the slot ledger and the served set, so
a namespace another surface owns renders nothing without declaring
anything, and a card for an uncomposed plugin is never dispatched.

packages/client/ui-plugin-config/tests/section.client.spec.tsx

@@ -20,6 +20,7 @@ import type { WebSearchCardProps } from '../src/client/WebSearchCard.tsx'
 import type { AgentLoopCardState } from '../src/client/agent-loop-store.ts'
 import type { BashCardState } from '../src/client/bash-store.ts'
 import type { CardFieldState, CardShell } from '../src/client/card-store.ts'
+import type { PluginConfigSectionState } from '../src/client/section-store.ts'
 import type { WebSearchCardState } from '../src/client/web-search-store.ts'
 import { en } from '../src/client/locales.ts'
 
@@ -46,11 +47,20 @@ function cardActions() {
   return { edit: vi.fn(), resetField: vi.fn(), save: vi.fn(), discard: vi.fn() }
 }
 
-function renderSection(cardCount: number, cards = 'cards') {
+/**
+ * Render the section over the namespaces it was told to dispatch, with `cards`
+ * standing in for the slot ledger: a key it names renders that text, and one
+ * it does not renders nothing, exactly as an unclaimed key does.
+ */
+function renderSection(namespaces: string[], cards: Record<string, string> = {}, loaded = true) {
+  const store = createSnapshotStore<PluginConfigSectionState>({ loaded, namespaces })
   const props = {
     t,
-    cardCount,
-    renderSlot: () => <li>{cards}</li>,
+    usePluginConfigSection: bindSnapshotSelector(store),
+    renderSlot: (_name: string, _owner: object, opts?: { entryKey?: string }) => {
+      const card = opts?.entryKey === undefined ? undefined : cards[opts.entryKey]
+      return card === undefined ? null : <li>{card}</li>
+    },
   } as unknown as PluginConfigSectionProps
   render(<PluginConfigSection {...props} />)
 }
@@ -70,21 +80,30 @@ function renderBash(state: Partial<BashCardState> = {}) {
 
 describe('PluginConfigSection', () => {
   it('says so when no plugin contributed a card', () => {
-    renderSection(0)
+    renderSection([], { bash: 'shell' })
 
     expect(screen.getByText(en.empty)).toBeTruthy()
-    expect(screen.queryByText('cards')).toBeNull()
+    expect(screen.queryByText('shell')).toBeNull()
   })
 
-  it('renders the card list once a plugin contributed one', () => {
-    renderSection(1)
+  it('withholds the empty line until the Host has answered once', () => {
+    // An unanswered read is not the statement that this deployment configures
+    // no plugin; saying it anyway would flash a wrong answer on every open.
+    renderSection([], { bash: 'shell' }, false)
 
-    expect(screen.getByText('cards')).toBeTruthy()
+    expect(screen.queryByText(en.empty)).toBeNull()
+    expect(screen.getByRole('heading', { name: en.title })).toBeTruthy()
+  })
+
+  it('dispatches one card per namespace, keyed by it', () => {
+    renderSection(['bash', 'agent-loop'], { bash: 'shell', 'agent-loop': 'loop' })
+
+    expect(screen.getAllByRole('listitem').map(item => item.textContent)).toEqual(['shell', 'loop'])
     expect(screen.queryByText(en.empty)).toBeNull()
   })
 
   it('leads with its own heading and intro', () => {
-    renderSection(1)
+    renderSection(['bash'], { bash: 'shell' })
 
     expect(screen.getByRole('heading', { name: en.title })).toBeTruthy()
     expect(screen.getByText(en.intro)).toBeTruthy()