feat(desktop): steer the live run from the composer

The desktop app could only queue while busy — `/steer` was in the palette
but had no first-class affordance, so the "nudge the agent mid-turn without
interrupting" lane was effectively unreachable.

Add a steer action to the composer: while busy with a text-only draft, a
steering-wheel button (and Cmd/Ctrl+Enter) injects the text into the live
turn via the `session.steer` RPC — the gateway folds it into the next tool
result so the model reads it on its next iteration. Plain Enter still queues.

steerPrompt returns false when the gateway has no live tool window (or the
RPC errors), and the composer re-queues the words so nothing is lost — the
same safety net as a plain queue.

apps/desktop/src/app/session/hooks/use-prompt-actions.test.tsx

@@ -41,6 +41,7 @@ function sessionInfo(overrides: Partial<SessionInfo> = {}): SessionInfo {
 }
 
 interface HarnessHandle {
+  steerPrompt: (text: string) => Promise<boolean>
   submitText: (text: string, options?: { attachments?: never[]; fromQueue?: boolean }) => Promise<boolean>
 }
 
@@ -88,8 +89,8 @@ function Harness({
   })
 
   useEffect(() => {
-    onReady({ submitText: actions.submitText })
-  }, [actions.submitText, onReady])
+    onReady({ steerPrompt: actions.steerPrompt, submitText: actions.submitText })
+  }, [actions.steerPrompt, actions.submitText, onReady])
 
   return null
 }
@@ -259,3 +260,57 @@ describe('usePromptActions submit / queue drain semantics', () => {
     expect(requestGateway).not.toHaveBeenCalledWith('prompt.submit', expect.anything())
   })
 })
+
+describe('usePromptActions steerPrompt', () => {
+  afterEach(() => {
+    cleanup()
+    vi.restoreAllMocks()
+  })
+
+  it('injects the trimmed text via session.steer and reports acceptance on a queued status', async () => {
+    const requestGateway = vi.fn(async () => ({ status: 'queued' }) as never)
+
+    let handle: HarnessHandle | null = null
+    render(<Harness onReady={h => (handle = h)} refreshSessions={async () => undefined} requestGateway={requestGateway} />)
+
+    const accepted = await handle!.steerPrompt('  nudge the run  ')
+
+    expect(accepted).toBe(true)
+    // Steer never starts a turn — it rides the live run via session.steer only.
+    expect(requestGateway).toHaveBeenCalledWith('session.steer', {
+      session_id: RUNTIME_SESSION_ID,
+      text: 'nudge the run'
+    })
+    expect(requestGateway).not.toHaveBeenCalledWith('prompt.submit', expect.anything())
+  })
+
+  it('reports rejection (so the caller queues) when the gateway has no live tool window', async () => {
+    const requestGateway = vi.fn(async () => ({ status: 'rejected' }) as never)
+
+    let handle: HarnessHandle | null = null
+    render(<Harness onReady={h => (handle = h)} refreshSessions={async () => undefined} requestGateway={requestGateway} />)
+
+    expect(await handle!.steerPrompt('too late')).toBe(false)
+  })
+
+  it('reports rejection (never throws) when the steer RPC errors', async () => {
+    const requestGateway = vi.fn(async () => {
+      throw new Error('agent does not support steer')
+    })
+
+    let handle: HarnessHandle | null = null
+    render(<Harness onReady={h => (handle = h)} refreshSessions={async () => undefined} requestGateway={requestGateway} />)
+
+    expect(await handle!.steerPrompt('boom')).toBe(false)
+  })
+
+  it('skips the RPC entirely for empty text', async () => {
+    const requestGateway = vi.fn(async () => ({ status: 'queued' }) as never)
+
+    let handle: HarnessHandle | null = null
+    render(<Harness onReady={h => (handle = h)} refreshSessions={async () => undefined} requestGateway={requestGateway} />)
+
+    expect(await handle!.steerPrompt('   ')).toBe(false)
+    expect(requestGateway).not.toHaveBeenCalled()
+  })
+})

apps/desktop/src/app/session/hooks/use-prompt-actions.ts

@@ -41,7 +41,13 @@ import {
   setYoloActive
 } from '@/store/session'
 
-import type { ClientSessionState, ImageAttachResponse, SessionTitleResponse, SlashExecResponse } from '../../types'
+import type {
+  ClientSessionState,
+  ImageAttachResponse,
+  SessionSteerResponse,
+  SessionTitleResponse,
+  SlashExecResponse
+} from '../../types'
 
 function blobToDataUrl(blob: Blob): Promise<string> {
   return new Promise((resolve, reject) => {
@@ -743,6 +749,37 @@ export function usePromptActions({
     }
   }, [activeSessionId, activeSessionIdRef, busyRef, requestGateway, updateSessionState])
 
+  // Steer = nudge the live turn without interrupting: the gateway appends the
+  // text to the next tool result so the model reads it on its next iteration
+  // (desktop parity with `/steer`). Returns false on reject (no live tool
+  // window) so the caller can fall back to queueing the words for the next turn.
+  const steerPrompt = useCallback(
+    async (rawText: string): Promise<boolean> => {
+      const text = rawText.trim()
+      const sessionId = activeSessionId || activeSessionIdRef.current
+
+      if (!text || !sessionId) {
+        return false
+      }
+
+      try {
+        const result = await requestGateway<SessionSteerResponse>('session.steer', { session_id: sessionId, text })
+
+        if (result?.status === 'queued') {
+          triggerHaptic('submit')
+          notify({ kind: 'success', title: 'Steered', message: text })
+
+          return true
+        }
+      } catch {
+        // Swallow — caller queues the text so nothing is lost.
+      }
+
+      return false
+    },
+    [activeSessionId, activeSessionIdRef, requestGateway]
+  )
+
   const reloadFromMessage = useCallback(
     async (parentId: string | null) => {
       if (!activeSessionId || $busy.get()) {
@@ -926,5 +963,13 @@ export function usePromptActions({
     [activeSessionIdRef, updateSessionState]
   )
 
-  return { cancelRun, editMessage, handleThreadMessagesChange, reloadFromMessage, submitText, transcribeVoiceAudio }
+  return {
+    cancelRun,
+    editMessage,
+    handleThreadMessagesChange,
+    reloadFromMessage,
+    steerPrompt,
+    submitText,
+    transcribeVoiceAudio
+  }
 }