feat(ui-models): schema-driven provider configuration page

The Models settings section joins llm.providers (the configurable
directory with live state), settings.describe (schemas, layered redacted
values, secret slots), and credentials.describe (value-free badges) into
provider rows with one editor card at a time. The editor renders the
provider's profile subtree through dsh-client-schema-form; the
credential-ref role mounts a control that shows configured/source state
and stores keys write-only through credentials.set. Apply without
removals merges a minimal patch (stored secrets outside it survive);
apply after a reset — and row deletion — replace the user section so
removals land. The client runtime bridges the three new host frames to
typed ctx events (settings/credentials/models changed), the page
refetches on any of them once loaded, and ui-model's per-session picker
directories reload on models/changed so a settings-born route appears in
open pickers without a reopen.

packages/client/ui-models/tests/invariant.spec.ts

@@ -17,7 +17,7 @@ describe('invariant companion', () => {
     expect(true).toBe(true) // reaching here without throw is the contract
   })
 
-  it('the section content column is intentionally empty this phase', () => {
-    expect(ModelsSection()).toBeNull()
+  it('renders null until the shell injects the section dependencies', () => {
+    expect(ModelsSection({})).toBeNull()
   })
 })