Feature: parallel tool execution

[jx-qiu]

Currently all tools called by the planner are executed sequentially:

with self._operation_lock:
    if self._active_operation is not None:
        return ToolResult(
            name=name,
            result={"error": "another tool operation is still active"},
        )
    operation = _ToolOperation()
    self._active_operation = operation

Read-only tools can be parallelized, while a tool that changes env state must be sequential and blocking. This dependency graph can be automated since all read-only tools are marked with @readonly annotations.