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/schema-form/src/index.ts

@@ -11,6 +11,6 @@ export type {
   SchemaFieldContext, SchemaFormLabels, SchemaFormProps, SchemaFormSecret,
 } from './SchemaForm.tsx'
 export {
-  deletePath, getPath, hasPath, nodeKind, rehydrateSchema, setPath, unionChoices, validateDraft,
+  deletePath, getPath, hasPath, nodeAtPath, nodeKind, rehydrateSchema, setPath, unionChoices, validateDraft,
 } from './model.ts'
 export type { NodeKind, SchemaNode } from './model.ts'

packages/client/schema-form/src/model.ts

@@ -78,6 +78,26 @@ export function unionChoices(node: SchemaNode): unknown[] {
   return (node.list ?? []).map(branch => (branch as { value?: unknown }).value)
 }
 
+/**
+ * Resolve the schema node at a settings path (the configurable-provider
+ * directory's `settingsPath` vocabulary): object properties by name, dict
+ * entries through `inner`. An unresolvable segment returns `undefined` so
+ * the caller falls back instead of rendering a wrong subtree.
+ * @param root - rehydrated section root node.
+ * @param path - key path from the section root.
+ * @returns the node describing that position, or `undefined`.
+ */
+export function nodeAtPath(root: SchemaNode, path: readonly string[]): SchemaNode | undefined {
+  let node: SchemaNode | undefined = root
+  for (const key of path) {
+    if (node === undefined) return undefined
+    if (node.type === 'object') node = (node.dict as Record<string, SchemaNode> | undefined)?.[key]
+    else if (node.type === 'dict' || node.type === 'array') node = node.inner as SchemaNode | undefined
+    else return undefined
+  }
+  return node
+}
+
 /**
  * Read a nested value by path.
  * @param value - root value (draft or fallback layer).