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/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()
   })