feat: auto-create artifacts from file_write tool output

When the agent writes files via the file_write tool, artifacts are now
automatically created in the artifact panel. The file extension determines
the artifact type (code/markdown/text) and syntax highlighting language.

This connects the existing ArtifactPanel UI to actual tool output data.

desktop/src/store/chat/streamStore.ts

@@ -34,6 +34,7 @@ import {
   resolveGatewayAgentId,
 } from './conversationStore';
 import { useMessageStore } from './messageStore';
+import { useArtifactStore } from './artifactStore';
 
 const log = createLogger('StreamStore');
 
@@ -297,6 +298,39 @@ export const useStreamStore = create<StreamState>()(
                   return { ...m, toolSteps: steps };
                 })
               );
+
+              // Auto-create artifact when file_write tool produces output
+              if (tool === 'file_write' && output) {
+                try {
+                  const parsed = JSON.parse(output);
+                  const filePath = parsed?.path || parsed?.file_path || '';
+                  const content = parsed?.content || '';
+                  if (filePath && content) {
+                    const fileName = filePath.split('/').pop() || filePath;
+                    const ext = fileName.split('.').pop()?.toLowerCase() || '';
+                    const typeMap: Record<string, 'code' | 'markdown' | 'text'> = {
+                      ts: 'code', tsx: 'code', js: 'code', jsx: 'code',
+                      py: 'code', rs: 'code', go: 'code', java: 'code',
+                      md: 'markdown', txt: 'text', json: 'code',
+                      html: 'code', css: 'code', sql: 'code', sh: 'code',
+                    };
+                    const langMap: Record<string, string> = {
+                      ts: 'typescript', tsx: 'typescript', js: 'javascript', jsx: 'javascript',
+                      py: 'python', rs: 'rust', go: 'go', java: 'java',
+                      html: 'html', css: 'css', sql: 'sql', sh: 'bash', json: 'json',
+                    };
+                    useArtifactStore.getState().addArtifact({
+                      id: `artifact_${Date.now()}`,
+                      name: fileName,
+                      content: typeof content === 'string' ? content : JSON.stringify(content, null, 2),
+                      type: typeMap[ext] || 'text',
+                      language: langMap[ext],
+                      createdAt: new Date(),
+                      sourceStepId: assistantId,
+                    });
+                  }
+                } catch { /* non-critical: artifact creation from tool output */ }
+              }
             } else {
               // toolStart: create new running step
               const step: ToolCallStep = {