Add an 'open in codespace' button to coding agent sessions

[joshspicer]

Add to the context menu at the end of the completed chat session.

[joshspicer]

Codespaces extension has github.codespaces.continueEditSessionInCodespace, though it reads the local environment's git ref for the codespace's ref. We're trying to avoid needing a checkout

https://github.com/github/codespaces-vscode/blob/main/src/codespaces/src/commands/CreateNewEnvironmentWithEditSession.ts#L38

[joshspicer]

ref https://vscodeteam.slack.com/archives/C01CJTEHNN7/p1756942841210689

[joshspicer]

Requires assistance from codespaces team