[Follow-up #255] Prune old unliked entries from Like aggregate state

[podclaude]

Context

Follow-up from PR #255: feat: add like/unlike system

Original Finding

The Like aggregate stores all historical likes including old unliked entries. For a power user who likes and unlikes many episodes over time, this grows unboundedly and gets loaded into memory on every command. Consider only storing currently-liked entries in the aggregate, or pruning old unliked entries from the checkpoint.

Acceptance Criteria

• Prune unliked entries from aggregate state (keep only active likes)
• Or trim on SnapshotLike to only store active likes in checkpoint
• Add test verifying state doesn't grow unboundedly