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

pnpm-lock.yaml

@@ -143,6 +143,9 @@ importers:
       '@deepseek-ai/dsh-client-ui-layout':
         specifier: workspace:^
         version: link:../../packages/client/ui-layout
+      '@deepseek-ai/dsh-client-ui-model':
+        specifier: workspace:^
+        version: link:../../packages/client/ui-model
       '@deepseek-ai/dsh-client-ui-models':
         specifier: workspace:^
         version: link:../../packages/client/ui-models
@@ -959,6 +962,27 @@ importers:
         specifier: ^18.2.0
         version: 18.3.1
 
+  packages/client/ui-model:
+    devDependencies:
+      '@deepseek-ai/dsh-client-connection':
+        specifier: workspace:^
+        version: link:../connection
+      '@deepseek-ai/dsh-client-runtime':
+        specifier: workspace:^
+        version: link:../runtime
+      '@deepseek-ai/dsh-client-ui-command':
+        specifier: workspace:^
+        version: link:../ui-command
+      '@deepseek-ai/dsh-client-ui-slash':
+        specifier: workspace:^
+        version: link:../ui-slash
+      '@deepseek-ai/dsh-invariants':
+        specifier: workspace:^
+        version: link:../../support/invariants
+      cordis:
+        specifier: ^4.0.0-rc.7
+        version: 4.0.0-rc.7(@cordisjs/plugin-include@1.0.4)(@cordisjs/plugin-loader@1.0.0-rc.5)
+
   packages/client/ui-models:
     devDependencies:
       '@deepseek-ai/dsh-client-locale':