feat: add ]f/[f navigation support in history mode

[esmuellert]

Summary

Add tab-wide ]f/[f keymaps that work from the diff pane in history mode, addressing user requests for keyboard navigation without switching focus.

Changes

• Single-file mode (:CodeDiff history %): ]f/[f navigate between commits
• Multi-file mode (:CodeDiff history): ]f/[f navigate between files within expanded commits

Files Modified

• lua/codediff/ui/view/keymaps.lua - Smart navigation based on mode
• lua/codediff/ui/view/init.lua - Keymap setup for history mode
• lua/codediff/ui/history/render.lua - Added commit navigation functions
• lua/codediff/ui/history/init.lua - Exported new functions
• README.md, doc/codediff.txt - Documentation updates

Testing

All existing tests pass.

Closes #187
Closes #208

[fredrikaverpil]

Unless I am missing something I still need to manually expand each commit so to move between files across commits?
I opened a POC on auto-expansions of commits here: #212