Use depth 5 imitation bootstrap

[k15z]

Summary

• Increase imitation bootstrap minimax depth from 3 to 5 for the clean run.
• Update the config comment to reflect the stronger-but-more-expensive target generation.

Validation

• cargo test -p hexchess-engine minimax::tests::depth3_returns_move
• uv run pytest training/test_config.py

Runtime note

• Started a worker-only depth-5 generation deployment on kevz-gpu with a runtime config override while this PR/image build is pending.