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

packages/host/apiproxy/tests/client-handler.spec.ts

@@ -48,8 +48,10 @@ function scriptedApi(overrides: {
       ...overrides.sessions,
     },
     host: {
-      describe: r => ok(r, { version: '0-test', cwd: '/t', attachedSessions: 0 }),
+      describe: r => ok(r, { version: '0-test', cwd: '/t', attachedSessions: 0, directoryPicker: 'browse' as const }),
       pickDirectory: r => ok(r, { path: null }),
+      listDirectory: r => ok(r, { path: '/t', home: '/t', crumbs: [], entries: [] }),
+      createDirectory: r => ok(r, { path: '/t/new' }),
       ...overrides.host,
     },
     workspace: {