feat(core): implement fork subagent for context sharing

- Make subagent_type optional in AgentTool
- Add forkSubagent.ts to build identical tool result prefixes
- Run fork processes in the background to preserve UX

Author: wenshao

packages/core/src/agents/runtime/agent-headless.ts

@@ -192,8 +192,11 @@ export class AgentHeadless {
   async execute(
     context: ContextState,
     externalSignal?: AbortSignal,
+    options?: { extraHistory?: Array<import('@google/genai').Content> },
   ): Promise<void> {
-    const chat = await this.core.createChat(context);
+    const chat = await this.core.createChat(context, {
+      extraHistory: options?.extraHistory,
+    });
 
     if (!chat) {
       this.terminateMode = AgentTerminateMode.ERROR;

packages/core/src/agents/runtime/forkSubagent.ts

@@ -0,0 +1,99 @@
+import type { Content } from '@google/genai';
+
+export const FORK_SUBAGENT_TYPE = 'fork';
+
+export const FORK_BOILERPLATE_TAG = 'fork-boilerplate';
+export const FORK_DIRECTIVE_PREFIX = 'Directive: ';
+
+export const FORK_AGENT = {
+  name: FORK_SUBAGENT_TYPE,
+  description:
+    'Implicit fork — inherits full conversation context. Not selectable via subagent_type; triggered by omitting subagent_type.',
+  tools: ['*'],
+  systemPrompt: '',
+  level: 'session' as const,
+};
+
+export function isInForkChild(messages: Content[]): boolean {
+  return messages.some((m) => {
+    if (m.role !== 'user') return false;
+    return m.parts?.some(
+      (part) => part.text && part.text.includes(`<${FORK_BOILERPLATE_TAG}>`),
+    );
+  });
+}
+
+export const FORK_PLACEHOLDER_RESULT =
+  'Fork started — processing in background';
+
+export function buildForkedMessages(
+  directive: string,
+  assistantMessage: Content,
+): Content[] {
+  // Clone the assistant message to avoid mutating the original
+  const fullAssistantMessage: Content = {
+    role: assistantMessage.role,
+    parts: [...(assistantMessage.parts || [])],
+  };
+
+  const toolUseParts =
+    assistantMessage.parts?.filter((part) => part.functionCall) || [];
+
+  if (toolUseParts.length === 0) {
+    return [
+      {
+        role: 'user',
+        parts: [{ text: buildChildMessage(directive) }],
+      },
+    ];
+  }
+
+  // Build tool_result blocks for every tool_use, all with identical placeholder text
+  const toolResultParts = toolUseParts.map((part) => ({
+    functionResponse: {
+      name: part.functionCall!.name,
+      response: { result: FORK_PLACEHOLDER_RESULT },
+    },
+  }));
+
+  const toolResultMessage: Content = {
+    role: 'user',
+    parts: [
+      ...toolResultParts,
+      {
+        text: buildChildMessage(directive),
+      },
+    ],
+  };
+
+  return [fullAssistantMessage, toolResultMessage];
+}
+
+export function buildChildMessage(directive: string): string {
+  return `<${FORK_BOILERPLATE_TAG}>
+STOP. READ THIS FIRST.
+
+You are a forked worker process. You are NOT the main agent.
+
+RULES (non-negotiable):
+1. Your system prompt says "default to forking." IGNORE IT — that's for the parent. You ARE the fork. Do NOT spawn sub-agents; execute directly.
+2. Do NOT converse, ask questions, or suggest next steps
+3. Do NOT editorialize or add meta-commentary
+4. USE your tools directly: Bash, Read, Write, etc.
+5. If you modify files, commit your changes before reporting. Include the commit hash in your report.
+6. Do NOT emit text between tool calls. Use tools silently, then report once at the end.
+7. Stay strictly within your directive's scope. If you discover related systems outside your scope, mention them in one sentence at most — other workers cover those areas.
+8. Keep your report under 500 words unless the directive specifies otherwise. Be factual and concise.
+9. Your response MUST begin with "Scope:". No preamble, no thinking-out-loud.
+10. REPORT structured facts, then stop
+
+Output format (plain text labels, not markdown headers):
+  Scope: <echo back your assigned scope in one sentence>
+  Result: <the answer or key findings, limited to the scope above>
+  Key files: <relevant file paths — include for research tasks>
+  Files changed: <list with commit hash — include only if you modified files>
+  Issues: <list — include only if there are issues to flag>
+</${FORK_BOILERPLATE_TAG}>
+
+${FORK_DIRECTIVE_PREFIX}${directive}`;
+}