feat: add fork assistant message to new session

Add fork button on assistant messages to start new execution session
Include synthetic meta-instruction when forking to explain context

packages/ui/src/lib/messages/executionMeta.ts

@@ -0,0 +1,8 @@
+export const EXECUTION_FORK_META_TEXT =
+    "This message comes from an AI assistant in another session. The user wants you to respond according to its content: " +
+    "if it is an implementation plan, your task is to implement that plan; " +
+    "if it is a conclusion or summary, your task is to verify it, explain whether you agree or disagree, and correct it if needed. " +
+    "Always clearly state what you understand your task to be, and wait for the user's approval of your conclusions before taking any further actions.";
+
+export const isExecutionForkMetaText = (text: string | null | undefined): boolean =>
+    typeof text === 'string' && text.trim() === EXECUTION_FORK_META_TEXT.trim();

packages/ui/src/lib/messages/messageText.ts

@@ -0,0 +1,13 @@
+import type { Part } from '@opencode-ai/sdk';
+
+type TextLikePart = Part & { text?: string; content?: string };
+
+export const flattenAssistantTextParts = (parts: Part[]): string => {
+    const textParts = parts
+        .filter((part): part is TextLikePart => part?.type === 'text')
+        .map((part) => (part.text || part.content || '').trim())
+        .filter((text) => text.length > 0);
+
+    const combined = textParts.join('\n');
+    return combined.replace(/\n\s*\n+/g, '\n');
+};