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/buffer.lua

@@ -115,7 +115,7 @@ local function setup_syntax(bufnr)
     vim.cmd([[
       syntax clear
       syntax match taskId /^\/\d\+\// conceal
-      syntax match taskHeader /^## .*$/ contains=taskId
+      syntax match taskHeader /^# .*$/ contains=taskId
       syntax match taskCheckbox /\[!\]/ contained containedin=taskLine
       syntax match taskLine /^\/\d\+\/- \[.\] .*$/ contains=taskId,taskCheckbox
     ]])
@@ -205,9 +205,8 @@ local function apply_extmarks(bufnr, line_meta)
       else
         icon, icon_hl = icons.pending, 'Normal'
       end
-      local icon_padded = icon .. '  '
       vim.api.nvim_buf_set_extmark(bufnr, task_ns, row, bracket_col, {
-        virt_text = { { icon_padded, icon_hl } },
+        virt_text = { { '[' .. icon .. ']', icon_hl } },
         virt_text_pos = 'overlay',
         priority = 100,
       })
@@ -218,7 +217,7 @@ local function apply_extmarks(bufnr, line_meta)
         hl_group = 'PendingHeader',
       })
       vim.api.nvim_buf_set_extmark(bufnr, task_ns, row, 0, {
-        virt_text = { { icons.header .. '  ', 'PendingHeader' } },
+        virt_text = { { icons.category .. ' ', 'PendingHeader' } },
         virt_text_pos = 'overlay',
         priority = 100,
       })

lua/pending/config.lua

@@ -2,7 +2,6 @@
 ---@field pending string
 ---@field done string
 ---@field priority string
----@field header string
 ---@field due string
 ---@field recur string
 ---@field category string
@@ -82,10 +81,9 @@ local defaults = {
   },
   sync = {},
   icons = {
-    pending = '-',
+    pending = ' ',
     done = 'x',
     priority = '!',
-    header = '>',
     due = '.',
     recur = '~',
     category = '#',

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

lua/pending/views.lua

@@ -133,7 +133,7 @@ function M.category_view(tasks)
       table.insert(lines, '')
       table.insert(meta, { type = 'blank' })
     end
-    table.insert(lines, '## ' .. cat)
+    table.insert(lines, '# ' .. cat)
     table.insert(meta, { type = 'header', category = cat })
 
     local all = {}