Work a single GitHub issue end to end — fetch its full context, branch, and start. Use when Josh points at an issue and wants it worked: 'let's look at issue 43', 'work on #43', 'start 43', 'pick up foo/bar#43', 'what's next?', 'grab the next issue', 'let's fix that bug'. Also use with no ref when he says 'let's get started' or 'what am I on?' — the ref resolves from the conversation or from the current git branch. Routing: if the issue has at least one open sub-issue it is a parent, so hand off to the `epic` skill with that ref; `epic` covers 'close out the epic', 'work through all of #40's children', 'do the whole epic'.