Allow old stories to be archived (or deleted)

[jimbojw]

Background:
Currently, stories represent a monotonically increasing list of directories under /stories. This is fine in many cases, but leads to two predictable problems:

1. Code bloat - Descriptions, logs and artifacts from completed stories are downloaded and present even when just the main programmatic content (/ai-guides etc.) is needed.
2. Search poisoning - IDEs and other tools may end up finding old story content that's not relevant to current tasks.

Recommendation:
Implement an optional scheme by which the highest used story number can be reliably determined, even if the directories for old stories have been deleted or moved.

Replacing old story directories with placeholder archive files would also work. E.g. the stories/s001-initial-project-setup dir becomes a stories/s001-initial-project-setup.archive.md file (or similar) which contains a description of what was once in that directory. This might offer a hybrid solution for discoverability without the whole story contents.