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

spec/icons_spec.lua

@@ -0,0 +1,59 @@
+require('spec.helpers')
+
+local config = require('pending.config')
+
+describe('icons', function()
+  before_each(function()
+    vim.g.pending = nil
+    config.reset()
+  end)
+
+  after_each(function()
+    vim.g.pending = nil
+    config.reset()
+  end)
+
+  it('has default icon values', function()
+    local icons = config.get().icons
+    assert.equals('○', icons.pending)
+    assert.equals('✓', icons.done)
+    assert.equals('●', icons.priority)
+    assert.equals('▸', icons.header)
+    assert.equals('·', icons.due)
+    assert.equals('↺', icons.recur)
+    assert.equals('#', icons.category)
+  end)
+
+  it('allows overriding individual icons', function()
+    vim.g.pending = { icons = { pending = '-', done = 'x' } }
+    config.reset()
+    local icons = config.get().icons
+    assert.equals('-', icons.pending)
+    assert.equals('x', icons.done)
+    assert.equals('●', icons.priority)
+    assert.equals('▸', icons.header)
+  end)
+
+  it('allows overriding all icons', function()
+    vim.g.pending = {
+      icons = {
+        pending = '-',
+        done = 'x',
+        priority = '!',
+        header = '>',
+        due = '@',
+        recur = '~',
+        category = '+',
+      },
+    }
+    config.reset()
+    local icons = config.get().icons
+    assert.equals('-', icons.pending)
+    assert.equals('x', icons.done)
+    assert.equals('!', icons.priority)
+    assert.equals('>', icons.header)
+    assert.equals('@', icons.due)
+    assert.equals('~', icons.recur)
+    assert.equals('+', icons.category)
+  end)
+end)