feat(customization): icons config, PendingTab, and demo infrastructure (#46)

* feat(config): add icons table with unicode defaults

* feat(buffer): render icon overlays from config.icons

Problem: status characters ([ ], [x], [!]) and metadata prefixes are
hardcoded literals with no user customization.

Solution: read config.icons in apply_extmarks and apply overlay
extmarks for checkboxes/headers, replace hardcoded recur ↺ with
icons.recur, and prefix due/category virt_text with configurable
icon characters.

* feat(plugin): add PendingTab command and <Plug>(pending-tab)

* docs: add icons config, PendingTab recipes, and demo infrastructure

Problem: icon customization and auto-start workflow are undocumented;
no demo asset exists for the README.

Solution: document pending.Icons in vimdoc with nerd font and ASCII
recipes, add PendingTab to commands and mappings, add open-on-startup
recipe, add demo-init.lua and demo.tape for VHS screenshot generation,
add assets/ directory, add README icons section and demo placeholder.

* ci: format

lua/pending/buffer.lua

@@ -143,6 +143,7 @@ end
 ---@param bufnr integer
 ---@param line_meta pending.LineMeta[]
 local function apply_extmarks(bufnr, line_meta)
+  local icons = config.get().icons
   vim.api.nvim_buf_clear_namespace(bufnr, task_ns, 0, -1)
   for i, m in ipairs(line_meta) do
     local row = i - 1
@@ -156,13 +157,13 @@ local function apply_extmarks(bufnr, line_meta)
       local due_hl = m.overdue and 'PendingOverdue' or 'PendingDue'
       local virt_parts = {}
       if m.show_category and m.category then
-        table.insert(virt_parts, { m.category, 'PendingHeader' })
+        table.insert(virt_parts, { icons.category .. ' ' .. m.category, 'PendingHeader' })
       end
       if m.recur then
-        table.insert(virt_parts, { '\u{21bb} ' .. m.recur, 'PendingRecur' })
+        table.insert(virt_parts, { icons.recur .. ' ' .. m.recur, 'PendingRecur' })
       end
       if m.due then
-        table.insert(virt_parts, { m.due, due_hl })
+        table.insert(virt_parts, { icons.due .. ' ' .. m.due, due_hl })
       end
       if m.file then
         local display = m.file:match('([^/]+:%d+)$') or m.file
@@ -185,12 +186,33 @@ local function apply_extmarks(bufnr, line_meta)
           hl_group = 'PendingDone',
         })
       end
+      local line = vim.api.nvim_buf_get_lines(bufnr, row, row + 1, false)[1] or ''
+      local bracket_col = (line:find('%[') or 1) - 1
+      local icon, icon_hl
+      if m.status == 'done' then
+        icon, icon_hl = icons.done, 'PendingDone'
+      elseif m.priority and m.priority > 0 then
+        icon, icon_hl = icons.priority, 'PendingPriority'
+      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_pos = 'overlay',
+        priority = 100,
+      })
     elseif m.type == 'header' then
       local line = vim.api.nvim_buf_get_lines(bufnr, row, row + 1, false)[1] or ''
       vim.api.nvim_buf_set_extmark(bufnr, task_ns, row, 0, {
         end_col = #line,
         hl_group = 'PendingHeader',
       })
+      vim.api.nvim_buf_set_extmark(bufnr, task_ns, row, 0, {
+        virt_text = { { icons.header .. '  ', 'PendingHeader' } },
+        virt_text_pos = 'overlay',
+        priority = 100,
+      })
     end
   end
 end

lua/pending/config.lua

@@ -1,3 +1,12 @@
+---@class pending.Icons
+---@field pending string
+---@field done string
+---@field priority string
+---@field header string
+---@field due string
+---@field recur string
+---@field category string
+
 ---@class pending.GcalConfig
 ---@field calendar? string
 ---@field credentials_path? string
@@ -38,6 +47,7 @@
 ---@field keymaps pending.Keymaps
 ---@field sync? pending.SyncConfig
 ---@field gcal? pending.GcalConfig
+---@field icons pending.Icons
 
 ---@class pending.config
 local M = {}
@@ -71,6 +81,15 @@ local defaults = {
     prev_task = '[t',
   },
   sync = {},
+  icons = {
+    pending = '○',
+    done = '✓',
+    priority = '●',
+    header = '▸',
+    due = '·',
+    recur = '↺',
+    category = '#',
+  },
 }
 
 ---@type pending.Config?