refactor: consolidate streaming text ownership and forward intent events

- Main process now always emits resolved content in message-delta events,
  making it the single authoritative text assembly point
- Renderer no longer imports or runs mergeStreamingText; uses main-process
  resolved content directly with simple append as defensive fallback
- Forward assistant-intent events through the session event pipeline; track
  currentIntent on SessionRecord, cleared on turn completion/cancellation
- Forward reasoning-delta events through the session event pipeline so
  renderers can consume incremental reasoning text
- Add SessionEventKind entries: assistant-intent, reasoning-delta
- Add SessionEventRecord fields: intent, reasoningId, reasoningDelta
- Remove dead applyTurnDelta, applyMessageReclassified, applyAgentActivity
  methods from AryxAppService (superseded by SessionTurnExecutor)
- Remove stale mergeStreamingText and appendRunActivityEvent imports from
  AryxAppService
- Add 5 regression tests: resolved content path, simple append fallback,
  intent set/clear lifecycle, duplicate suppression, intent preservation
  during status transitions

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/renderer/lib/sessionWorkspace.ts

@@ -2,7 +2,6 @@ import type { SessionEventRecord } from '@shared/domain/event';
 import { upsertSessionRunRecord } from '@shared/domain/runTimeline';
 import type { ChatMessageRecord, SessionRecord } from '@shared/domain/session';
 import type { WorkspaceState } from '@shared/domain/workspace';
-import { mergeStreamingText } from '@shared/utils/streamingText';
 
 export function applySessionEventWorkspace(
   current: WorkspaceState | undefined,
@@ -45,6 +44,8 @@ function applySessionEvent(session: SessionRecord, event: SessionEventRecord): S
       return applyMessageReclassifiedEvent(session, event);
     case 'run-updated':
       return applyRunUpdatedEvent(session, event);
+    case 'assistant-intent':
+      return applyAssistantIntentEvent(session, event);
     default:
       return session;
   }
@@ -63,6 +64,7 @@ function applyStatusEvent(session: SessionRecord, event: SessionEventRecord): Se
     ...session,
     status: event.status,
     lastError: event.status === 'error' ? session.lastError : undefined,
+    currentIntent: event.status === 'idle' ? undefined : session.currentIntent,
     updatedAt: event.occurredAt,
   };
 }
@@ -86,14 +88,19 @@ function applyMessageDeltaEvent(session: SessionRecord, event: SessionEventRecor
     return session;
   }
 
-  const resolvedContent = event.content ?? event.contentDelta ?? '';
   const messageIndex = session.messages.findIndex((message) => message.id === event.messageId);
   if (messageIndex >= 0) {
     const existing = session.messages[messageIndex];
+    // The main process resolves content authoritatively; prefer event.content
+    // (full assembled text) and fall back to simple append for backward compat.
+    const nextContent =
+      event.content !== undefined
+        ? event.content
+        : existing.content + (event.contentDelta ?? '');
     const nextMessage: ChatMessageRecord = {
       ...existing,
       authorName: event.authorName ?? existing.authorName,
-      content: event.content ?? mergeStreamingText(existing.content, resolvedContent),
+      content: nextContent,
       pending: true,
     };
 
@@ -114,6 +121,8 @@ function applyMessageDeltaEvent(session: SessionRecord, event: SessionEventRecor
     };
   }
 
+  const resolvedContent = event.content ?? event.contentDelta ?? '';
+
   // Auto-complete any previously pending assistant messages so only
   // the new message shows the "Thinking" indicator.
   const completedMessages = session.messages.map((message) =>
@@ -217,3 +226,15 @@ function applyRunUpdatedEvent(session: SessionRecord, event: SessionEventRecord)
     updatedAt: event.occurredAt,
   };
 }
+
+function applyAssistantIntentEvent(session: SessionRecord, event: SessionEventRecord): SessionRecord {
+  if (!event.intent || session.currentIntent === event.intent) {
+    return session;
+  }
+
+  return {
+    ...session,
+    currentIntent: event.intent,
+    updatedAt: event.occurredAt,
+  };
+}