feat: merge tool calls with their updates into single unified view

Instead of showing tool_call and tool_call_update as separate events,
now merge them by toolCallId into a single MergedToolCall that updates
as new information comes in.

**Changes:**
- Added MergedToolCall type with start/end timestamps
- Renamed groupTextChunks to groupActivities with tool call merging logic
- Track tool calls by ID and merge all updates into single entry
- Replace ToolCallItem with MergedToolCallItem component
- AgentStreamView now uses groupActivities for unified grouping

**User Experience:**
- Each tool call appears once and updates in place
- Status changes from pending → in_progress → completed
- Input/output merge as they become available
- Cleaner, less noisy activity view

Generated with [Claude Code](https://claude.ai/code)
via [Happy](https://happy.engineering)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

packages/app/src/components/agent-stream-view.tsx

@@ -5,7 +5,7 @@ import { StyleSheet } from 'react-native-unistyles';
 import type { AgentStatus } from '@server/server/acp/types';
 import type { SessionNotification } from '@agentclientprotocol/sdk';
 import { AgentActivityItem } from './agent-activity';
-import { parseSessionUpdate, groupTextChunks, type AgentActivity } from '@/types/agent-activity';
+import { parseSessionUpdate, groupActivities, type AgentActivity } from '@/types/agent-activity';
 
 export interface AgentStreamViewProps {
   agentId: string;
@@ -94,8 +94,8 @@ export function AgentStreamView({
       })
       .filter((activity): activity is AgentActivity => activity !== null);
 
-    // Group consecutive text chunks
-    return groupTextChunks(activities);
+    // Group consecutive text chunks and merge tool calls
+    return groupActivities(activities);
   }, [updates]);
 
   // Auto-scroll to bottom when new updates arrive