Adapt session model selection to the slash/input/session architecture

- host trio kept on the merged api-proxy: session.models (provider-grouped
  advisory directory), session.selectModel (validated provider, advisory
  model), installAgentLlmTarget threaded through create/resume/ensureSession;
  the gateway declares the llm inject it reads
- history no longer piggybacks modelTarget: the current target travels on
  session.models alone (the /model popup is the sole consumer)
- new @deepseek-ai/dsh-client-ui-model plugin: /model popupSelect over the
  wire — options load the directory (group label in the detail column,
  provider-local failures listed inline), onSelect routes selectModel;
  failures ride the popup shell's error/retry surface
- ModelSelector package, conversation.composer.control slot, and the
  Session-side modelSelection state machine are removed: model selection
  belongs to the /model popup; the named conversation.input.model seat
  stays empty until a control-seat entry is designed for it

packages/client/ui-model/src/css-modules.d.ts

@@ -0,0 +1,6 @@
+declare module '*.module.css' {
+  const classes: Record<string, string>
+  export default classes
+}
+
+declare module '*.css'

packages/client/ui-model/src/index.ts

@@ -0,0 +1,9 @@
+/**
+ * Model selection plugin, node half. Pure UI plugin: the empty apply exists
+ * so the plugin appears in the host cordis.yml / Loader; the browser half
+ * ships via exports["./client"], discovered through the package.json
+ * dshClient declaration.
+ */
+
+/** Host plugin body — no host-side behavior for this surface plugin. */
+export function apply(): void {}

packages/client/ui-model/src/invariant.ts

@@ -0,0 +1,31 @@
+/**
+ * Package-owned invariant companion for `@deepseek-ai/dsh-client-ui-model`.
+ * @module @deepseek-ai/dsh-client-ui-model/invariant
+ */
+
+/* jscpd:ignore-start */
+import type { Context } from 'cordis'
+import type { InvariantInstaller } from '@deepseek-ai/dsh-invariants'
+
+const PACKAGE_NAME = '@deepseek-ai/dsh-client-ui-model'
+
+/** Cordis companion plugin name. */
+export const name = 'client-ui-model-invariant'
+/** Service required before the companion can reserve package ownership. */
+export const inject = ['invariants']
+
+/**
+ * No runtime invariant: a single command contribution registration whose disposal is
+ * proven by the HMR-safety spec — it emits no cordis events and owns no
+ * cross-plugin mutable state.
+ */
+const install: InvariantInstaller = () => {}
+
+/**
+ * Register this package's invariant companion.
+ * @param ctx - Cordis context carrying the invariant service.
+ * @returns the installed registration's disposer after setup succeeds.
+ */
+export const apply = (ctx: Context): Promise<() => void> =>
+  Promise.resolve(ctx.invariants.register(PACKAGE_NAME, install))
+/* jscpd:ignore-end */