feat: listen to runCommand requests from LSP

The runCommand request asks the editor to open an integrated terminal to
run a command within it.

The TerminalManager re-uses integrated terminals keyed to workflow IDs,
this ensures that an entire workflow or agentic chat session reuses a
terminal and users have a quick summary of what was run.

This functionality relies on the user to have shell integration set up.
This appears to be pretty automatic. The vscode documentation suggests a
3 second timeout to await for the shell integration to be present, but
I'm not sure if that is too long.

In the event that the shell integration is not present, we reject the
request and expect the language server to run the command itself.

These terminals are marked as transient, so they are not resurrected
upon re-opening VS Code.