feat(web): pick a session's agent preset from the composer

A seat in the composer tool row, left of the model select, showing the preset
THIS session runs — its own recorded one, not the deployment default, because a
resumed session runs what it was created with. `SessionSummary` carries
`agentPreset` for that, alongside `cwd` and `origin`.

The switch exists only while the conversation has not started. After the first
turn the seat becomes a plain label rather than a disabled menu: a greyed
control reads as "temporarily unavailable", when in fact the choice is gone for
good. The seat never asks in that state, and the host refuses independently
with `agent-preset-locked`, so a stale client cannot slip one past it.

A rejected switch restores the previous value and surfaces the host's message
rather than leaving the seat showing something the session is not running.

packages/client/ui-conversation/src/client/apply.ts

@@ -228,6 +228,7 @@ export function apply(ctx: Context): void {
     children: {
       'conversation.input.plan': { kind: 'single', scope: 'session' },
       'conversation.input.model': { kind: 'single', scope: 'session' },
+      'conversation.input.agentPreset': { kind: 'single', scope: 'session' },
     },
     inject: (sessionId: SessionId | undefined): ComposerBarInjected => {
       if (sessionId === undefined) {

packages/client/ui-conversation/src/client/contract/slots.ts

@@ -102,6 +102,12 @@ declare module '@deepseek-ai/dsh-client-ui-slots' {
      * empty-until-registered contract as the plan seat.
      */
     'conversation.input.model': { kind: 'single'; scope: 'session'; owner: InputControlOwnerProps }
+    /**
+     * The agent-preset seat in the composer tool row, left of the model.
+     * Same empty-until-registered contract as the other two; its owner
+     * decides on its own whether the session may still switch.
+     */
+    'conversation.input.agentPreset': { kind: 'single'; scope: 'session'; owner: InputControlOwnerProps }
   }
 
   /**
@@ -325,7 +331,7 @@ export interface InputControlOwnerProps {
 /** Full composer-bar props: standard kit & owner share & control-seat render share & injected share (hooks bound) & locale seat. */
 export type ComposerBarProps =
   PropsRuntime<'conversation.composer.bar'>
-  & PropsRenderSlots<'conversation.input.plan' | 'conversation.input.model'>
+  & PropsRenderSlots<'conversation.input.plan' | 'conversation.input.model' | 'conversation.input.agentPreset'>
   & InjectFace<ComposerBarInjected>
   & PropsLocale<'conversation'>
 

packages/client/ui-conversation/src/client/skeleton/InputBar.tsx

@@ -512,6 +512,7 @@ export function InputBar({
           </div>
           <div className={css.trailing}>
             {rightItems}
+            {renderSlot('conversation.input.agentPreset', { locked })}
             {renderSlot('conversation.input.model', { locked })}
             <ContextMeter useProjection={useProjection} t={t} />
             {/* {machineBusy && <span className={css.pending} data-input-pending aria-label="处理中" />} */}

packages/client/ui-conversation/tests/input-bar.spec.tsx

@@ -686,13 +686,15 @@ describe('strips and variants', () => {
 })
 
 describe('command launcher chrome and control seats', () => {
-  it('renders the command launcher; the Access chip is absent without the permissions projection; plan/model seats render EMPTY without entries (B ruling)', () => {
+  it('renders the command launcher; the Access chip is absent without the permissions projection; the control seats render EMPTY without entries (B ruling)', () => {
     const { view, slotCalls } = bench()
     expect(view.getByLabelText('命令')).toBeTruthy()
     // Capability absent (no projection value): the chip renders nothing.
     expect(view.queryByLabelText(/^访问模式/)).toBeNull()
-    // Both seats dispatched, nothing rendered.
-    expect(slotCalls.map(c => c.key)).toEqual(['conversation.input.plan', 'conversation.input.model'])
+    // Every seat dispatched, nothing rendered.
+    expect(slotCalls.map(c => c.key)).toEqual([
+      'conversation.input.plan', 'conversation.input.agentPreset', 'conversation.input.model',
+    ])
     expect(view.queryByLabelText('Plan mode')).toBeNull()
     expect(view.queryByLabelText('Model')).toBeNull()
   })