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

plugin/pending.lua

@@ -304,3 +304,13 @@ end)
 vim.keymap.set('n', '<Plug>(pending-add-here)', function()
   require('pending').add_here()
 end)
+
+vim.keymap.set('n', '<Plug>(pending-tab)', function()
+  vim.cmd.tabnew()
+  require('pending').open()
+end)
+
+vim.api.nvim_create_user_command('PendingTab', function()
+  vim.cmd.tabnew()
+  require('pending').open()
+end, {})