[FEATURE]: Clarify VS Code extension name in manual install section of the docs

[baldassarreFe]

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

The automatic install wasn't working for me, so I searched the extension marketplace for "opencode". I got multiple results and some of them are sketchy. Even the least sketchy one was still a bit sketchy because I didn't recognize the publisher "sst". The docs should include a link to the legit extension to avoid users installing a bad one.

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• Opencode plugin for VS Code install instructions ambiguous #10517: Same report — VS Code extension marketplace search for "opencode" returns many results with no clear indication of which is official, and the manual install docs don't link directly to the extension.