Use the shell phase type in the conversation header

Why: The phase type has to be consumed by the screen that branches on it, or the unused-export ratchet fails.
Evidence: knip stays at the existing baseline after the conversation annotates ShellPhase.
Impact: No behavior change.
Refs: feature/runstate-shell
Co-authored-by: Cursor <cursoragent@cursor.com>

src/components/Conversation.tsx

@@ -8,7 +8,7 @@ import { api } from '../api'
 import { store, useStore, activeRun, type LiveState, type ContextObs } from '../store'
 import { scrollRatio, scrollTopForRatio } from '../scrollsync'
 import { liveToMessage, livePhaseLabel, showProcessInline } from '../live'
-import { deriveShellPhase, isShellBusy, showLiveBubble } from '../runstate'
+import { deriveShellPhase, isShellBusy, showLiveBubble, type ShellPhase } from '../runstate'
 import type { Message, Part, HITLCard, ActivityCard, Artifact } from '../contracts'
 import { UIRenderer, InlineConfirmCard, hasOpenConfirm, CardAnchor } from '@ui'
 import { docBridge } from './Workbench'
@@ -434,7 +434,7 @@ export function Conversation() {
   artifactCache = Object.fromEntries(artifacts.map(a => [a.id, a]))
   cardCache = Object.fromEntries(cards.map(c => [c.id, c]))
 
-  const phase = deriveShellPhase({
+  const phase: ShellPhase = deriveShellPhase({
     posting,
     openRun: !!activeRun(runs),
     liveRunId: live?.runId ?? null,