refactor: consolidate utility functions into shared package

Move duplicate isObject, asString, asNumber, and safeStringify functions from multiple modules into a centralized shared/src/utils.ts module and update imports across cli, server, and web packages. This eliminates code duplication and improves maintainability.

cli/src/agent/backends/acp/AcpMessageHandler.ts

@@ -1,5 +1,6 @@
 import type { AgentMessage, PlanItem } from '@/agent/types';
-import { asString, deriveToolName, isObject } from '@/agent/utils';
+import { asString, isObject } from '@hapi/protocol';
+import { deriveToolName } from '@/agent/utils';
 import { ACP_SESSION_UPDATE_TYPES } from './constants';
 
 function normalizeStatus(status: unknown): 'pending' | 'in_progress' | 'completed' | 'failed' {

cli/src/agent/backends/acp/AcpSdkBackend.ts

@@ -1,5 +1,5 @@
 import type { AgentBackend, AgentMessage, AgentSessionConfig, PermissionRequest, PermissionResponse, PromptContent } from '@/agent/types';
-import { asString, isObject } from '@/agent/utils';
+import { asString, isObject } from '@hapi/protocol';
 import { AcpStdioTransport, type AcpStderrError } from './AcpStdioTransport';
 import { AcpMessageHandler } from './AcpMessageHandler';
 import { logger } from '@/ui/logger';

cli/src/agent/utils.ts

@@ -1,10 +1,4 @@
-export function isObject(value: unknown): value is Record<string, unknown> {
-    return Boolean(value) && typeof value === 'object';
-}
-
-export function asString(value: unknown): string | null {
-    return typeof value === 'string' ? value : null;
-}
+import { isObject } from '@hapi/protocol';
 
 export function deriveToolName(input: {
     title?: string | null;