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/host/apiproxy/src/api/rpc.schema.ts

@@ -41,6 +41,7 @@ export const rpcErrorSchema: z.ZodType<RpcError> = z.discriminatedUnion('code',
   z.object({ code: z.literal('workspace-invalid-path'), message: z.string(), details: z.object({ path: z.string() }) }),
   z.object({ code: z.literal('workspace-name-conflict'), message: z.string(), details: z.object({ name: z.string() }) }),
   z.object({ code: z.literal('workspace-move-invalid'), message: z.string(), details: z.object({ workspaceId: z.string(), sessionId: z.string(), beforeSessionId: z.string().optional() }) }),
+  z.object({ code: z.literal('model-unavailable'), message: z.string(), details: z.object({ provider: z.string(), model: z.string() }) }),
   z.object({ code: z.literal('agent-busy'), message: z.string(), details: z.object({ reason: z.string() }) }),
   z.object({ code: z.literal('internal'), message: z.string(), details: z.object({}) }),
 ]) as unknown as z.ZodType<RpcError>

packages/host/apiproxy/src/api/rpc.ts

@@ -38,6 +38,7 @@ export interface RpcErrorDetailsMap {
   'workspace-invalid-path': { path: string }
   'workspace-name-conflict': { name: string }
   'workspace-move-invalid': { workspaceId: string; sessionId: SessionId; beforeSessionId?: SessionId }
+  'model-unavailable': { provider: string; model: string }
   'agent-busy': { reason: string }
   'internal': {}
 }

packages/host/apiproxy/src/api/sessions.schema.ts

@@ -128,7 +128,6 @@ export const historyEntrySchema = z.object({
 export const sessionHistoryValueSchema = z.object({
   events: z.array(historyEntrySchema),
   hasMore: z.boolean(),
-  modelTarget: modelTargetSchema,
 }) satisfies z.ZodType<Wire<ResponseValue<'session.history'>>>
 
 /** session.models request payload. */

packages/host/apiproxy/src/api/sessions.ts

@@ -129,7 +129,7 @@ export interface SessionsApi {
    * rebuilds the surface from the events with the shared fold.
    */
   history(request: RpcRequest<{ sessionId: SessionId; beforeSeq?: number; maxMessages?: number }>):
-  Promise<RpcResponse<{ events: HistoryEntry[]; hasMore: boolean; modelTarget: ModelTarget }>>
+  Promise<RpcResponse<{ events: HistoryEntry[]; hasMore: boolean }>>
 
   /** Reads a fresh advisory model directory for this session. Provider lookups run independently. */
   models(request: RpcRequest<{ sessionId: SessionId }>): Promise<RpcResponse<SessionModels>>