Return `yield` result from `pty::spawn()`

There is no way atm to get anything from the `pty::spawn()` block. Though sometimes it'd be nice to pass something to the calling scope. Easiest way is to return `Any` type. Maybe a bit better way is to return `Bolt::Result` type.

Start by locating the pty::spawn() implementation and how it handles the block result. Compare the proposed Any and Bolt::Result return shapes with the existing project conventions, then define the expected calling-scope behavior and verify it with the relevant tests or a focused reproduction.

Written by the indexing model from the issue text.