refactor: consolidate and expand ls function family

- Upgrade ls to long listing (-l -a) and drop the standalone l and lS functions in favor of abbreviations
- Remove llm (ambiguous name, redundant with ltr)
- Add lsr (reversed time oneline), lss (size-sorted), lD (dirs-only), lx (extension-sorted)
- Enhance ltr with --all and age color-scale gradient
- Wire up ls abbreviations: l, lS, lsR, lX, lT, lsT
- Update README functions table and add Listing abbreviations section

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

functions/ltr.fish

@@ -3,9 +3,9 @@
 
 function ltr --description 'Reversed time-sorted listing'
     if which eza >/dev/null 2>&1
-        eza --long --sort=modified --reverse --icons --color=auto --hyperlink $argv
+        eza --long --all --sort=modified --reverse --icons --hyperlink --color=auto --color-scale=age --color-scale-mode=gradient $argv
     else if which lsd >/dev/null 2>&1
-        lsd -ltr $argv
+        lsd --long --all --sort=time --reverse --color=auto --hyperlink=always $argv
     else
         command ls --color=auto -ltr $argv
     end