refactor(host): rename the directory-picker dialog backend and kind to native

The browse interaction also presents a dialog (the in-app modal), so 'dialog'
failed to discriminate the two capability kinds; 'native' names where the
chooser runs. Package directory-picker-dialog -> directory-picker-native, kind
'dialog' -> 'native', with every seam/gateway/client/doc reference updated and
the seam Agent Note's naming rationale rewritten to match.

packages/client/ui-workspace/tests/workspace-browser.spec.tsx

@@ -60,7 +60,7 @@ function mount(overrides: Partial<WorkspaceBrowserProps> = {}) {
     insertSessionBefore: vi.fn(async () => {}),
     createWorkspace: vi.fn(async () => workspace('created', [])),
     pickDirectory: vi.fn(async () => null),
-    directoryPickerKind: vi.fn(async () => 'dialog' as const),
+    directoryPickerKind: vi.fn(async () => 'native' as const),
     ...overrides,
   }
   const view = render(<WorkspaceBrowser {...props} />)