Fix lazygit diff pager width for delta

[michaelbarton]

Summary

• Switch lazygit pager config from legacy paging.pager to the modern pagers array format
• Add --width={{columnWidth}} to the delta pager command so delta uses the lazygit panel width instead of the full terminal width
• Fixes side-by-side diff rendering and prevents line wrapping/overflow in the lazygit diff panel
• Fix black formatting to use project's --line-length=100 setting (was causing CI failure)

Supersedes #65 (closed due to CI formatting failure).

Changes

# Before (legacy format, no width)
paging:
  colorArg: always
  pager: delta --paging=never

# After (modern format, correct panel width)
pagers:
  - pager: delta --paging=never --width={{columnWidth}}

Test plan

• Open lazygit in a repo with unstaged changes
• Verify delta renders diffs correctly in the diff panel
• Verify side-by-side mode uses the correct panel width (no overflow/wrapping)
• Try cycling pagers with the pipe key (new capability from pagers array format)

https://claude.ai/code/session_01MnCD6D42cqYCZA7aJwJEEN