Feature Request: Built-in Actions for Agent Media Management

## Summary

Add built-in actions that allow agents to list, reference, and prune media during execution. This enables agents to have programmatic control over accumulated media content.

No implementation files or tests are named. Start by mapping the existing built-in action system and the agent execution path, then trace how media is accumulated and referenced. Done means the list, reference, and prune actions have agreed behavior and are integrated into agent execution.

Written by the indexing model from the issue text.