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.

doc/pending.txt

@@ -356,6 +356,10 @@ COMMANDS                                                    *pending-commands*
     Equivalent to the `U` buffer-local key (see |pending-mappings|). Up to 20
     levels of undo are persisted across sessions.
 
+                                                            *:PendingTab*
+:PendingTab
+    Open the task buffer in a new tab.
+
 ==============================================================================
 MAPPINGS                                                    *pending-mappings*
 
@@ -494,6 +498,9 @@ All motions support count: `3]]` jumps three headers forward. `]]` and
     to the current buffer's file and the cursor's line number.
     See |pending-file-token|.
 
+<Plug>(pending-tab)                             *<Plug>(pending-tab)*
+    Open the task buffer in a new tab. See |:PendingTab|.
+
 Example configuration: >lua
     vim.keymap.set('n', '<leader>t', '<Plug>(pending-open)')
     vim.keymap.set('n', '<leader>T', '<Plug>(pending-toggle)')
@@ -679,6 +686,16 @@ Fields: ~
                          automatically. New configs should use `sync.gcal`
                          instead. See |pending.GcalConfig|.
 
+    {icons}              (table)                          *pending.Icons*
+                         Icon characters displayed in the buffer. Fields:
+                           {pending}   Uncompleted task icon. Default: '○'
+                           {done}      Completed task icon. Default: '✓'
+                           {priority}  Priority task icon. Default: '●'
+                           {header}    Category header prefix. Default: '▸'
+                           {due}       Due date prefix. Default: '·'
+                           {recur}     Recurrence prefix. Default: '↺'
+                           {category}  Category label prefix. Default: '#'
+
 ==============================================================================
 LUA API                                                          *pending-api*
 
@@ -860,6 +877,41 @@ directly, or disable `a_category` in `keymaps` and handle it via a
 `vim.b.miniai_config` entry that returns a linewise region if mini.ai's
 spec allows it in your version.
 
+Nerd font icons: >lua
+    vim.g.pending = {
+      icons = {
+        pending  = '',
+        done     = '',
+        priority = '',
+        header   = '',
+        due      = '',
+        recur    = '󰁯',
+        category = '',
+      },
+    }
+<
+
+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()
+        vim.cmd.PendingTab()
+      end,
+    })
+<
 ==============================================================================
 GOOGLE CALENDAR                                                 *pending-gcal*