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/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?