feat(client): configure host-plane plugins from a settings section

The section knows no namespace: it declares `settings.plugin.item` and
renders whatever cards were registered into it, so a plugin that ships a
browser half owns its card and its controls. The three cards here cover the
host-plane sections this deployment exposes.

A field shows its effective value and, when the raw user layer carries it, an
override badge and a reset that clears it back to the composition layer.
Controls commit on blur and Enter rather than per keystroke, which would burn
namespace revisions and race its own reads. The search key is the one value
that never rides a response: the card reports only whether one is configured
and writes it through the credentials domain, addressed by the reference the
section names.

A card renders nothing while its namespace is unavailable — a deployment that
does not compose the owning plugin should show no trace of it rather than a
disabled card the user cannot act on.

packages/client/README.md

@@ -32,6 +32,7 @@ The browser side of the dsh web GUI: shell boot, browser-host communication, sha
 | [`ui-model/`](ui-model/README.md) | Provides model selection in conversation surfaces. |
 | [`ui-permission/`](ui-permission/README.md) | Configures default permissions and switches the current session's access. |
 | [`ui-plan/`](ui-plan/README.md) | Presents active plan-mode status and its exit control. |
+| [`ui-plugin-config/`](ui-plugin-config/README.md) | The Plugins settings section: host-plane plugin configuration as expandable cards. |
 | [`ui-question/`](ui-question/README.md) | Presents interactive questions requested by the agent. |
 | [`ui-agent-preset/`](ui-agent-preset/README.md) | Selects a session's agent preset and authors preset compositions. |
 | [`ui-settings/`](ui-settings/README.md) | Hosts the settings interface and its extension areas. |