docs: add SIGIL Protocol integration example for custom tools

[MyMolt]

Description

Added a recommendation and code snippet demonstrating how to use the Sigil Protocol middleware (@sigil_gate) to secure custom tools in CrewAI against prompt injection and data exfiltration.

This is a minor but highly valuable documentation addition for enterprise users building tools that interact with untrusted inputs or critical systems.

Related Issues

N/A

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Note

Low Risk
Documentation-only change with no runtime or API impact; risk is limited to potential confusion if the SIGIL import/decorator usage changes.

Overview
Adds a new docs section, “Securing Custom Tools with SIGIL”, to the custom tools guide.

The section recommends applying the @sigil_gate decorator to BaseTool subclasses and includes a short code snippet showing how to import and wrap a tool to scan inputs/outputs for threat patterns before execution.

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