feat: add local and remote repository diff views

src/main.rs

@@ -40,6 +40,9 @@ Commands: :w [PATH], :q, :q!, :wq [PATH], :plugins, :PLUGIN [ARGS]
 Buffers: :edit PATH, :buffers, :buffer N, :bnext, :bprev, :bdelete[!] [N]
          :q refuses any dirty buffer; :wq saves active and quits only if all are clean
 Markdown links: :follow_link (primary caret; local files only)
+Diff: :diff (Normal); choose Before then After; Working checkout means disk, not unsaved buffers
+      Tab/Shift+Tab categories/focus; type to search IDs/refs; arrows navigate/scroll; Esc back/close
+      Remote refs/history load into temporary comparison repos without changing active refs/checkout
 
 Ruby config: $XDG_CONFIG_HOME/vamp/config.rb or ~/.config/vamp/config.rb
 Set VAMP_RUBY to choose the Ruby executable.