refactor(icons): unify category/header icon and use checkbox overlays

Problem: `header` and `category` were separate icons for the same
concept. The icon overlay replaced `[ ]` with a bare character,
hiding the markdown checkbox syntax. Header format `## ` produced
a double-space with single-char icons.

Solution: merge `header` into `category` (one icon for both header
lines and EOL labels). Overlay renders `[icon]` preserving bracket
syntax. Change header line format from `## ` to `# ` so the
2-char overlay (`# `) maps cleanly.

lua/pending/diff.lua

@@ -59,8 +59,8 @@ function M.parse_buffer(lines)
           lnum = i,
         })
       end
-    elseif line:match('^## (.+)$') then
-      current_category = line:match('^## (.+)$')
+    elseif line:match('^# (.+)$') then
+      current_category = line:match('^# (.+)$')
       table.insert(result, { type = 'header', category = current_category, lnum = i })
     end
   end