feat(shell): add fzf inline picker bound to @@

Adds __fzf_inline_picker function and @@ keybinding (Emacs + all Vi
modes). Opens an fzf session and injects the selected item at the
cursor position in the current command line. Includes SYNOPSIS/
DESCRIPTION doc block per project conventions. Updates README with
new binding in the User Bindings table.

conf.d/key_bindings.fish

@@ -54,6 +54,7 @@ function fish_user_key_bindings
     bind ctrl-alt-u _replace_command_token
     type -q qalc && bind ctrl-alt-= _qalc_eval
     bind ctrl-enter _smart_execute
+    bind @@ __fzf_inline_picker
 
     # Set bindings for all Vi modes:
     # 'default' is Vi-Command, 'insert' is Vi-Insert, 'visual' is Vi-Visual
@@ -63,5 +64,6 @@ function fish_user_key_bindings
         bind --mode $mode ctrl-alt-u _replace_command_token
         type -q qalc && bind --mode $mode ctrl-alt-= _qalc_eval
         bind --mode $mode ctrl-enter _smart_execute
+        bind --mode $mode @@ __fzf_inline_picker
     end
 end