Detach Sokoban from quest loop

[wjt]

Currently when you complete a quest and bring the threads you collected back to the Eternal Loom, you then must complete a trio of sokoban puzzles. Although the “rejoin the threads” theme of the puzzle matches the surrounding game, it is an awkward fit for the end of a quest, and is a sharp jump in difficulty

@manuq added a debug flag to skip these while prototyping a new quest-ending sequence where you retell your adventures to a crowd of townies.

Let's go further and move the puzzles away from the Eternal Loom to a sewing machine, which we have an asset for already. I suggest:

1. Store the number of threads banked at the Loom in the game state.
2. When you interact with the sewing machine, you can play one sokoban per banked thread, in order rather than random.
3. For now they are just for fun, but in future they could give you a reward.

[wjt]

I'm attaching this to the Demo milestone because the objective of the tutorial quest is to get the player to Fray's End in the first place, and the end-of-quest sokobans are a particularly bad fit in that case.