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.

doc/pending.txt

@@ -663,14 +663,18 @@ Fields: ~
                          table. Currently only `gcal` is built-in.
 
     {icons}              (table)                          *pending.Icons*
-                         Icon characters displayed in the buffer. Fields:
-                           {pending}   Uncompleted task icon. Default: '-'
-                           {done}      Completed task icon. Default: 'x'
-                           {priority}  Priority task icon. Default: '!'
-                           {header}    Category header prefix. Default: '>'
+                         Icon characters displayed in the buffer. The
+                         {pending}, {done}, and {priority} characters
+                         appear inside brackets (`[icon]`) as an overlay
+                         on the checkbox. The {category} character
+                         prefixes both header lines and EOL category
+                         labels. Fields:
+                           {pending}   Pending task character. Default: ' '
+                           {done}      Done task character. Default: 'x'
+                           {priority}  Priority task character. Default: '!'
                            {due}       Due date prefix. Default: '.'
                            {recur}     Recurrence prefix. Default: '~'
-                           {category}  Category label prefix. Default: '#'
+                           {category}  Category prefix. Default: '#'
 
 ==============================================================================
 STORE RESOLUTION                                    *pending-store-resolution*
@@ -847,10 +851,6 @@ Event-driven statusline refresh: >lua
 Nerd font icons: >lua
     vim.g.pending = {
       icons = {
-        pending  = '',
-        done     = '',
-        priority = '',
-        header   = '',
         due      = '',
         recur    = '󰁯',
         category = '',
@@ -858,20 +858,6 @@ Nerd font icons: >lua
     }
 <
 
-ASCII fallback icons: >lua
-    vim.g.pending = {
-      icons = {
-        pending  = '-',
-        done     = 'x',
-        priority = '!',
-        header   = '>',
-        due      = '@',
-        recur    = '~',
-        category = '+',
-      },
-    }
-<
-
 Open tasks in a new tab on startup: >lua
     vim.api.nvim_create_autocmd('VimEnter', {
       callback = function()