Propagate workspace roots into subagent sessions

Subagents (task tool) now inherit the parent session's
additionalDirectories via ToolSession → ExecutorOptions →
CreateAgentSessionOptions, so delegated agents see the same
<workspace-roots> block and can read/grep/glob added roots.

Co-authored-by: oh-my-pi <https://omp.sh>

packages/coding-agent/src/task/structured-subagent.ts

@@ -371,6 +371,7 @@ function buildExecutorOptions(
 	const enableMCP = !policy.planMode && (session.enableMCP ?? true);
 	return {
 		cwd: session.cwd,
+		additionalDirectories: session.additionalDirectories,
 		agent: policy.effectiveAgent,
 		task: renderSubagentPrompt(request.assignment),
 		assignment: request.assignment.trim(),