Default Hunk pane to diff main

[RestartDK]

Summary

• change Scatterer's default Hunk pane command from bare hunk to hunk diff main
• document Hunk theme behavior and the lack of a Tokyo Night Day built-in
• bump the Herdr plugin manifest to 0.1.10

Verification

• cargo fmt --check
• cargo test
• cargo clippy -- -D warnings