feat(host): directory-picker capability seam with dialog and browse backends

The web GUI's folder picking was hardwired to one interaction: a native
OS chooser compiled into the gateway, unusable for remote deployments
and swappable only by editing apiproxy source.

Directory picking becomes a three-package capability seam in
packages/host: ctx.directoryPicker returns a discriminated capability —
dialog (the extracted native chooser; host-display only) or browse
(new: one-level listing + child creation over Node stdlib, hidden flags
host-stamped, symlinks followed, ancestry crumbs; remote-capable). The
gateway injects the seam, advertises the kind via
host.describe.directoryPicker, serves host.listDirectory /
host.createDirectory under browse, and answers
directory-picker-unavailable across kinds. cordis.yml is the swap
point; apps/cli keeps dialog mounted, so behavior is unchanged until
the in-app browser PR flips the default. The connection fixture serves
a deterministic browse tree; WorkspacesService gains the browse calls
the browser UI will drive. Decision record:
.agents/notes/implemented/architecture/2026-07-28-directory-picker-capability-seam.md

scripts/gen-cordis-catalog.ts

@@ -226,6 +226,7 @@ const TYPE_LINK_EXEMPTIONS: Readonly<Record<string, string>> = {
   BashEnvContributor: 'service-local extension type is owned by packages/bash/tool-bash/src/index.ts',
   BashEnvVariableInfo: 'service-local metadata type is owned by packages/bash/tool-bash/src/index.ts',
   CompactAgentContext: 'compaction service input is owned by packages/compact/compact/src/index.ts',
+  DirectoryPickerCapability: 'picker interaction contract is owned by packages/host/directory-picker/README.md',
   CreateAgentOptions: 'agent creation contract is owned by packages/core/agent/README.md',
   Domain: 'domain interface is owned by packages/storage/storage-domain/README.md',
   DomainChanged: 'event-local snapshot is owned by packages/storage/storage-domain/src/events.ts',