Allow /statusline and /title slash commands during active turns

[canvrno-oai]

• Marks /title and /statusline as available during active tasks.
• Extends the existing slash-command availability test coverage to include these commands alongside /goal.

[canvrno-oai]

@codex review

[etraut-openai]

I like this change in general. I tested it manually, and it seems to work fine.

One scenario that I tested was an approval prompt that appears while the /title or /statusline menu is displayed. The approval prompt temporarily replaces the menu, which makes sense. Once the approval prompt is dismissed, the menu reappears. It was a little jarring to have the approval prompt suddenly appear, but I think this situation will be sufficiently rare that it's not a problem.