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

scripts/demo-init.lua

@@ -0,0 +1,39 @@
+vim.opt.runtimepath:prepend(vim.fn.getcwd())
+local tmpdir = vim.fn.tempname()
+vim.fn.mkdir(tmpdir, 'p')
+
+vim.g.pending = {
+  data_path = tmpdir .. '/tasks.json',
+  icons = {
+    pending = '○',
+    done = '✓',
+    priority = '●',
+    header = '▸',
+    due = '·',
+    recur = '↺',
+    category = '#',
+  },
+}
+
+local store = require('pending.store')
+store.load()
+
+local today = os.date('%Y-%m-%d')
+local yesterday = os.date('%Y-%m-%d', os.time() - 86400)
+local tomorrow = os.date('%Y-%m-%d', os.time() + 86400)
+
+store.add({
+  description = 'Finish quarterly report',
+  category = 'Work',
+  due = tomorrow,
+  recur = 'monthly',
+  priority = 1,
+})
+store.add({ description = 'Review pull requests', category = 'Work' })
+store.add({ description = 'Update deployment docs', category = 'Work', status = 'done' })
+store.add({ description = 'Buy groceries', category = 'Personal', due = today })
+store.add({ description = 'Call dentist', category = 'Personal', due = yesterday, priority = 1 })
+store.add({ description = 'Read chapter 5', category = 'Personal' })
+store.add({ description = 'Learn a new language', category = 'Someday' })
+store.add({ description = 'Plan hiking trip', category = 'Someday' })
+store.save()