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/directory-picker/tests/seam.spec.ts

@@ -0,0 +1,35 @@
+/** Contract behavior the seam itself owns: registration identity and typed failures. */
+
+import { describe, expect, it } from 'vitest'
+import { Context } from 'cordis'
+import { DirectoryPicker, DirectoryPickerError } from '../src/index.ts'
+import type { DirectoryPickerCapability } from '../src/index.ts'
+
+/** Minimal concrete backend: all a subclass owes the abstract class is capability(). */
+class StubPicker extends DirectoryPicker {
+  private readonly stub: DirectoryPickerCapability = { kind: 'dialog', pick: async () => null }
+  capability(): DirectoryPickerCapability {
+    return this.stub
+  }
+}
+
+describe('DirectoryPicker seam', () => {
+  it('registers a subclass as ctx.directoryPicker and leaves with its fiber', async () => {
+    const ctx = new Context()
+    const fiber = ctx.plugin(StubPicker)
+    await fiber.await()
+    expect(ctx.get('directoryPicker')).toBeInstanceOf(StubPicker)
+    expect(ctx.get('directoryPicker')!.capability().kind).toBe('dialog')
+    await fiber.dispose()
+    expect(ctx.get('directoryPicker')).toBeUndefined()
+  })
+
+  it('carries the business code and subject path on DirectoryPickerError', () => {
+    const failure = new DirectoryPickerError('directory-exists', '/home/u/x', '/home/u/x already exists')
+    expect(failure.name).toBe('DirectoryPickerError')
+    expect(failure.code).toBe('directory-exists')
+    expect(failure.path).toBe('/home/u/x')
+    expect(failure.message).toContain('already exists')
+    expect(failure).toBeInstanceOf(Error)
+  })
+})