refactor(icons): ascii defaults, checkbox overlays, and cleanup (#57)

* docs: remove unnecessary mini.ai recipe from vimdoc

Problem: the `*pending-mini-ai*` section assumed mini.ai intercepts
buffer-local `at`/`it`/`aC`/`iC` mappings, requiring a manual
`vim.b.miniai_config` workaround.

Solution: remove the section. Neovim's keymap resolver already
prioritizes longer buffer-local mappings over mini.ai's global
`a`/`i` handlers — no recipe needed.

* refactor(icons): unify category/header icon and use checkbox overlays

Problem: `header` and `category` were separate icons for the same
concept. The icon overlay replaced `[ ]` with a bare character,
hiding the markdown checkbox syntax. Header format `## ` produced
a double-space with single-char icons.

Solution: merge `header` into `category` (one icon for both header
lines and EOL labels). Overlay renders `[icon]` preserving bracket
syntax. Change header line format from `## ` to `# ` so the
2-char overlay (`# `) maps cleanly.

* ci: remove empty `assets/` placeholder

spec/icons_spec.lua

@@ -15,45 +15,42 @@ describe('icons', function()
 
   it('has default icon values', function()
     local icons = config.get().icons
-    assert.equals('-', icons.pending)
+    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)
 
   it('allows overriding individual icons', function()
-    vim.g.pending = { icons = { pending = '○', done = '✓' } }
+    vim.g.pending = { icons = { pending = '*', done = '+' } }
     config.reset()
     local icons = config.get().icons
-    assert.equals('○', icons.pending)
-    assert.equals('✓', icons.done)
+    assert.equals('*', icons.pending)
+    assert.equals('+', icons.done)
     assert.equals('!', icons.priority)
-    assert.equals('>', icons.header)
+    assert.equals('#', icons.category)
   end)
 
   it('allows overriding all icons', function()
     vim.g.pending = {
       icons = {
         pending = '-',
-        done = 'x',
-        priority = '!',
-        header = '>',
+        done = '+',
+        priority = '*',
         due = '@',
-        recur = '~',
-        category = '+',
+        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.done)
+    assert.equals('*', icons.priority)
     assert.equals('@', icons.due)
-    assert.equals('~', icons.recur)
-    assert.equals('+', icons.category)
+    assert.equals('^', icons.recur)
+    assert.equals('&', icons.category)
   end)
 end)