feat: Suggestion for documentation improvement

### Issue Description

I have a suggestion I was trying to play with Security agent and was going through the following documentation : https://github.com/microsoft/hve-core/blob/main/docs/agents/security/README.md

In this documentation link above there was no mention of installing HVE Core - Security Extension. It took me a lot of time to figure out that I need to install this extension in order to use the agent.

I think it would be a good idea to enhance a Pre-reqs section in the documentation to have all the required extension needed for given agents. The pre-req section only mention "The Security Planner agent installed and enabled in your VS Code workspace." There is no mention of having the correct VS code extension installed.

### Additional Context

_No response_

Open docs/agents/security/README.md and review its existing prerequisites section. Update the section so it names the HVE Core - Security Extension required for the Security Planner agent. Done when a reader can identify and install the required VS Code extension before using the agent.

Written by the indexing model from the issue text.