fix: legacy diff options not working due to merge order (#142)

The backward compatibility mapping for `vertical_split` and
`open_in_current_tab` options was not working because it checked
if the merged config had nil values, but after merging with defaults,
these fields were already populated.

Fixed by unconditionally applying the legacy option mappings when
the legacy options are present. This ensures backward compatibility
for users still using the old option names.

Fixes:
- vertical_split=false now correctly sets layout="horizontal"
- open_in_current_tab=false now correctly sets open_in_new_tab=true

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <noreply@anthropic.com>

Author: ddzero2c

lua/claudecode/config.lua

@@ -205,12 +205,12 @@ function M.apply(user_config)
   -- Backward compatibility: map legacy diff options to new fields if provided
   if config.diff_opts then
     local d = config.diff_opts
-    -- Map vertical_split -> layout (only if layout not explicitly set)
-    if d.layout == nil and type(d.vertical_split) == "boolean" then
+    -- Map vertical_split -> layout (legacy option takes precedence)
+    if type(d.vertical_split) == "boolean" then
       d.layout = d.vertical_split and "vertical" or "horizontal"
     end
-    -- Map open_in_current_tab -> open_in_new_tab (invert; only if not explicitly set)
-    if d.open_in_new_tab == nil and type(d.open_in_current_tab) == "boolean" then
+    -- Map open_in_current_tab -> open_in_new_tab (legacy option takes precedence)
+    if type(d.open_in_current_tab) == "boolean" then
       d.open_in_new_tab = not d.open_in_current_tab
     end
   end