Allow sidebar extensions to open links in new sidebar tabs

Fairly self-explanatory, I have an extension I'm building with a customisable set of links, and I'd like those links to be able to be opened in a copilot browser tab, like github.com links can, rather than an external browser, like all other links currently do.

Start by tracing how sidebar extensions currently handle links, comparing that behavior with github.com links that open in Copilot browser tabs. Done means customizable links from sidebar extensions open in a Copilot browser tab instead of the external browser.

索引モデルが issue の本文から書いたものです。