fix(host): derive the picker capability union from a merge-extensible map

ds-review-bot round 1: the seam documented a merge-extensible union but
shipped a closed alias, and the gateway schema rejected any kind beyond
dialog/browse — a third backend could neither implement the seam nor be
advertised. The union now derives from an augmentable DirectoryPickerCapabilities
map, host.describe.directoryPicker preserves unknown wire kinds, and the
browse fixture applies listDirectory's root special case so creating under
'/' no longer mints a '//name' identity.

packages/host/apiproxy/src/api/host.schema.ts

@@ -17,7 +17,9 @@ export const hostDescribeValueSchema = z.object({
   provider: z.string().optional(),
   model: z.string().optional(),
   attachedSessions: z.number().int().nonnegative(),
-  directoryPicker: z.union([z.literal('dialog'), z.literal('browse')]),
+  // Open string, not a literal union: unknown kinds must survive the wire so
+  // a merge-added capability can advertise (the client hides the affordance).
+  directoryPicker: z.string(),
 }) satisfies z.ZodType<Wire<ResponseValue<'host.describe'>>>
 
 /** host.pickDirectory request payload (empty object literal). */