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

lua/pending/config.lua

@@ -2,7 +2,6 @@
 ---@field pending string
 ---@field done string
 ---@field priority string
----@field header string
 ---@field due string
 ---@field recur string
 ---@field category string
@@ -82,10 +81,9 @@ local defaults = {
   },
   sync = {},
   icons = {
-    pending = '-',
+    pending = ' ',
     done = 'x',
     priority = '!',
-    header = '>',
     due = '.',
     recur = '~',
     category = '#',