style(plan-diff): refine modified-block visual — amber gutter, no fill

Drop the yellow background fill from .plan-diff-modified and keep only a
softened amber left border. Added/removed blocks remain loud (full fill +
strong border) because add/remove are block-scope events — the whole
block matters. Modify is a word-scope event — the individual changed
words carry loud inline red/green highlights, and a block-level fill
would compete with that inline work. The amber gutter at 75% opacity now
reads as a quiet "look inside, the change is in the text" marker that
sits coherently with the rest of the palette.

For provenance purposes, this commit was AI assisted.

Author: backnotprop

packages/editor/index.css

@@ -149,18 +149,21 @@ pre code.hljs .hljs-code {
 }
 
 /* Clean diff view - modified content (mix of additions and deletions in one
-   block, rendered inline via word-level diff). Amber/yellow border matches
-   the GitHub / VSCode convention that green=add, red=remove, yellow=both. */
+   block, rendered inline via word-level diff).
+   Deliberate asymmetry with added/removed: add/remove are BLOCK-scope events
+   — the whole block matters, so a loud fill is the right signal. Modify is
+   a WORD-scope event — the words matter, and the inline red-struck /
+   green-highlighted word markers already grab attention. A block-level fill
+   would compete with that inline work; an amber gutter on a normal
+   background says "look inside, the change is in the text" while staying
+   consistent with the green/red/yellow diff convention. */
 .plan-diff-modified {
-  border-left: 3px solid var(--warning);
-  background: oklch(from var(--warning) l c h / 0.06);
+  border-left: 3px solid oklch(from var(--warning) l c h / 0.75);
+  background: transparent;
   padding-left: 0.75rem;
   border-radius: 0 0.25rem 0.25rem 0;
   margin: 0.25rem 0;
 }
-.light .plan-diff-modified {
-  background: oklch(from var(--warning) l c h / 0.06);
-}
 
 /* Clean diff view - unchanged (dimmed) */
 .plan-diff-unchanged {