Store Claude SDK sessions in .warden/sessions

[dcramer]

Summary

The Claude SDK stores session data in its own directory, and we currently have no way to disable that. We should move/copy the session data from Claude's directory into .warden/sessions so we own and control the session artifacts.

Motivation

• Keep all Warden-related artifacts in one place (.warden/)
• Avoid relying on Claude SDK's internal storage location which could change
• Enable downstream tooling that inspects or replays session data

Approach

• After a Claude SDK run completes, move the session file from Claude's storage directory to .warden/sessions/
• Organize by run timestamp or session ID
• Ensure .warden/sessions is gitignored by default