fix(acp): defer auto-connect until historical session id resolves

When reopening a historical conversation, the ACP auto-connect fired before
the detail fetch resolved external_id, so the backend fell back to
session/new and the agent lost prior context. Gate both the auto-connect
effect and the focus-triggered reconnect on the session id being ready.

src/components/conversations/conversation-detail-panel.tsx

@@ -202,8 +202,6 @@ const ConversationTabView = memo(function ConversationTabView({
   const [hasSentMessage, setHasSentMessage] = useState(false)
 
   const hasPersistedConversation = dbConversationId != null
-  const canAutoConnect =
-    hasPersistedConversation || (agentsLoaded && usableAgentCount > 0)
 
   // Expose the runtime session key to the tab so the aux panel (Diff sidebar)
   // can look up live turns even before the DB conversation is created.
@@ -266,6 +264,17 @@ const ConversationTabView = memo(function ConversationTabView({
   }, [effectiveSessionStats, isActive, setSessionStats])
 
   const externalId = detail?.summary.external_id ?? undefined
+  // For persisted conversations opened from the sidebar, wait until the
+  // session's external_id has been resolved before auto-connecting.
+  // Otherwise the auto-connect effect fires with sessionId=undefined and
+  // the backend falls back to session/new, orphaning the historical
+  // context. cline doesn't support session resume, so it connects
+  // immediately regardless.
+  const awaitingHistoricalSessionId =
+    hasPersistedConversation && selectedAgent !== "cline" && detailLoading
+  const canAutoConnect =
+    (hasPersistedConversation || (agentsLoaded && usableAgentCount > 0)) &&
+    !awaitingHistoricalSessionId
   const draftStorageKey = useMemo(() => {
     if (dbConversationId != null) {
       return buildConversationDraftStorageKey(dbConversationId)

src/hooks/use-connection-lifecycle.ts

@@ -264,6 +264,10 @@ export function useConnectionLifecycle({
   }, [removeTask, clearSelectorTask])
 
   const handleFocus = useCallback(() => {
+    // Respect the caller's readiness gate — e.g. historical conversations
+    // set isActive=false until the session's external_id resolves, to
+    // avoid connecting with sessionId=undefined and orphaning context.
+    if (!isActive) return
     touchActivity(contextKey)
     if (!status || status === "disconnected" || status === "error") {
       setLastAutoConnectError(null)
@@ -274,6 +278,7 @@ export function useConnectionLifecycle({
       })
     }
   }, [
+    isActive,
     agentType,
     workingDir,
     sessionId,