Finish work

**Is your feature request related to a problem? Please describe.**
when i start work on an issues i click start work and all is good. but i have no way to "finish work" i use the statusbar label to help me remember what I'm working on (for example when i arrive on monday) and right now i don't see a way to clear the active issue

**Describe the solution you'd like**
if i've already started work on an issue, replace the "start work" button witha "Finish work" one

**Describe alternatives you've considered**
AFAIK when i start work on another issue it always replaces the active one. i could just keep doing that

Start by locating the existing VS Code “Start work” command, its statusbar label, and the active-issue state. Trace how starting another issue replaces the current one, then make the finish action clear that state and update the statusbar and button so no issue remains active.

Written by the indexing model from the issue text.