Implement artifact storage size limits and cleanup mechanism

## Problem
There are currently no size limits on artifact storage accumulation. With 90-day retention, this could lead to unbounded storage growth.

## Solution
- Add size limits for artifact storage
- Implement cleanup mechanisms for old artifacts
- Document retention policies more clearly
- Consider automatic purging of artifacts older than retention period

## Related
PR #2 review feedback

Review the feedback from PR #2 first, then locate how artifact storage and the 90-day retention policy are currently handled. Define the size limits, cleanup and purging behavior, and clearer retention documentation; done means bounded storage, expired artifacts are removed, and the policy is documented.

Written by the indexing model from the issue text.