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/package.json

@@ -0,0 +1,61 @@
+{
+  "name": "@deepseek-ai/dsh-client-ui-model",
+  "description": "Model selection: the /model popupSelect over session.models / session.selectModel",
+  "version": "0.0.1",
+  "private": true,
+  "type": "module",
+  "main": "lib/index.js",
+  "types": "lib/types/index.d.ts",
+  "exports": {
+    ".": {
+      "types": "./lib/types/index.d.ts",
+      "default": "./lib/index.js"
+    },
+    "./invariant": {
+      "types": "./lib/types/invariant.d.ts",
+      "default": "./lib/invariant.js"
+    },
+    "./client": {
+      "types": "./lib/types/client/index.d.ts",
+      "default": "./lib/client.js"
+    },
+    "./src/*": "./src/*",
+    "./package.json": "./package.json"
+  },
+  "dshClient": {
+    "inject": [
+      "@deepseek-ai/dsh-client-runtime",
+      "@deepseek-ai/dsh-client-ui-command"
+    ],
+    "platform": "web"
+  },
+  "scripts": {
+    "bundle": "tsdown",
+    "watch": "tsdown --watch"
+  },
+  "license": "BSD-3-Clause",
+  "peerDependencies": {
+    "@deepseek-ai/dsh-client-connection": "^0.0.1",
+    "@deepseek-ai/dsh-client-runtime": "^0.0.1",
+    "@deepseek-ai/dsh-client-ui-command": "^0.0.1",
+    "@deepseek-ai/dsh-client-ui-slash": "^0.0.1",
+    "@deepseek-ai/dsh-invariants": "^0.0.1",
+    "cordis": "^4.0.0-rc.7"
+  },
+  "devDependencies": {
+    "@deepseek-ai/dsh-client-connection": "workspace:^",
+    "@deepseek-ai/dsh-client-runtime": "workspace:^",
+    "@deepseek-ai/dsh-client-ui-command": "workspace:^",
+    "@deepseek-ai/dsh-client-ui-slash": "workspace:^",
+    "@deepseek-ai/dsh-invariants": "workspace:^",
+    "cordis": "^4.0.0-rc.7"
+  },
+  "files": [
+    "lib/index.js",
+    "lib/invariant.js",
+    "lib/client.js",
+    "lib/types/**/*.d.ts",
+    "lib/types/**/*.d.ts.map",
+    "src"
+  ]
+}

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 */

packages/client/ui-model/tsconfig.json

@@ -0,0 +1,30 @@
+{
+  "extends": "../../../tsconfig.base.client.json",
+  "compilerOptions": {
+    "rootDir": "src",
+    "outDir": "lib/types"
+  },
+  "include": [
+    "src"
+  ],
+  "references": [
+    {
+      "path": "../../../vendor/cordis"
+    },
+    {
+      "path": "../connection"
+    },
+    {
+      "path": "../runtime"
+    },
+    {
+      "path": "../ui-command"
+    },
+    {
+      "path": "../ui-slash"
+    },
+    {
+      "path": "../../support/invariants"
+    }
+  ]
+}

packages/client/ui-model/tsdown.config.ts

@@ -0,0 +1,3 @@
+import { clientBundle } from '../tsdown.client.ts'
+
+export default clientBundle('@deepseek-ai/dsh-client-ui-model', ['lib/types/index.js', 'lib/types/invariant.js'])