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

README.md

@@ -26,18 +26,18 @@ luarocks install pending.nvim
 
 ## Icons
 
-pending.nvim renders task status and metadata using configurable icon characters. The defaults are ASCII-only (no unicode or nerd font required):
+All display characters are configurable. Defaults produce markdown-style checkboxes (`[ ]`, `[x]`, `[!]`):
 
 ```lua
 vim.g.pending = {
   icons = {
-    pending = '-', done = 'x', priority = '!',
-    header = '>', due = '.', recur = '~', category = '#',
+    pending = ' ', done = 'x', priority = '!',
+    due = '.', recur = '~', category = '#',
   },
 }
 ```
 
-See `:help pending.Icons` for unicode and nerd font examples.
+See `:help pending.Icons` for nerd font examples.
 
 ## Acknowledgements