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/diff_spec.lua

@@ -21,11 +21,11 @@ describe('diff', function()
   describe('parse_buffer', function()
     it('parses headers and tasks', function()
       local lines = {
-        '## School',
+        '# School',
         '/1/- [ ] Do homework',
         '/2/- [!] Read chapter 5',
         '',
-        '## Errands',
+        '# Errands',
         '/3/- [ ] Buy groceries',
       }
       local result = diff.parse_buffer(lines)
@@ -44,7 +44,7 @@ describe('diff', function()
 
     it('handles new tasks without ids', function()
       local lines = {
-        '## Inbox',
+        '# Inbox',
         '- [ ] New task here',
       }
       local result = diff.parse_buffer(lines)
@@ -56,7 +56,7 @@ describe('diff', function()
 
     it('inline cat: token overrides header category', function()
       local lines = {
-        '## Inbox',
+        '# Inbox',
         '/1/- [ ] Buy milk cat:Work',
       }
       local result = diff.parse_buffer(lines)
@@ -67,7 +67,7 @@ describe('diff', function()
 
     it('extracts rec: token from buffer line', function()
       local lines = {
-        '## Inbox',
+        '# Inbox',
         '/1/- [ ] Take trash out rec:weekly',
       }
       local result = diff.parse_buffer(lines)
@@ -76,7 +76,7 @@ describe('diff', function()
 
     it('extracts rec: with completion mode', function()
       local lines = {
-        '## Inbox',
+        '# Inbox',
         '/1/- [ ] Water plants rec:!daily',
       }
       local result = diff.parse_buffer(lines)
@@ -86,7 +86,7 @@ describe('diff', function()
 
     it('inline due: token is parsed', function()
       local lines = {
-        '## Inbox',
+        '# Inbox',
         '/1/- [ ] Buy milk due:2026-03-15',
       }
       local result = diff.parse_buffer(lines)
@@ -99,7 +99,7 @@ describe('diff', function()
   describe('apply', function()
     it('creates new tasks from buffer lines', function()
       local lines = {
-        '## Inbox',
+        '# Inbox',
         '- [ ] First task',
         '- [ ] Second task',
       }
@@ -116,7 +116,7 @@ describe('diff', function()
       s:add({ description = 'Delete me' })
       s:save()
       local lines = {
-        '## Inbox',
+        '# Inbox',
         '/1/- [ ] Keep me',
       }
       diff.apply(lines, s)
@@ -132,7 +132,7 @@ describe('diff', function()
       s:add({ description = 'Original' })
       s:save()
       local lines = {
-        '## Inbox',
+        '# Inbox',
         '/1/- [ ] Renamed',
       }
       diff.apply(lines, s)
@@ -146,7 +146,7 @@ describe('diff', function()
       t.modified = '2020-01-01T00:00:00Z'
       s:save()
       local lines = {
-        '## Inbox',
+        '# Inbox',
         '/1/- [ ] Renamed',
       }
       diff.apply(lines, s)
@@ -160,7 +160,7 @@ describe('diff', function()
       s:add({ description = 'Original' })
       s:save()
       local lines = {
-        '## Inbox',
+        '# Inbox',
         '/1/- [ ] Original',
         '/1/- [ ] Copy of original',
       }
@@ -174,7 +174,7 @@ describe('diff', function()
       s:add({ description = 'Moving task', category = 'Inbox' })
       s:save()
       local lines = {
-        '## Work',
+        '# Work',
         '/1/- [ ] Moving task',
       }
       diff.apply(lines, s)
@@ -187,7 +187,7 @@ describe('diff', function()
       s:add({ description = 'Stable task', category = 'Inbox' })
       s:save()
       local lines = {
-        '## Inbox',
+        '# Inbox',
         '/1/- [ ] Stable task',
       }
       diff.apply(lines, s)
@@ -203,7 +203,7 @@ describe('diff', function()
       s:add({ description = 'Pay bill', due = '2026-03-15' })
       s:save()
       local lines = {
-        '## Inbox',
+        '# Inbox',
         '/1/- [ ] Pay bill',
       }
       diff.apply(lines, s)
@@ -214,7 +214,7 @@ describe('diff', function()
 
     it('stores recur field on new tasks from buffer', function()
       local lines = {
-        '## Inbox',
+        '# Inbox',
         '- [ ] Take out trash rec:weekly',
       }
       diff.apply(lines, s)
@@ -228,7 +228,7 @@ describe('diff', function()
       s:add({ description = 'Task', recur = 'daily' })
       s:save()
       local lines = {
-        '## Todo',
+        '# Todo',
         '/1/- [ ] Task rec:weekly',
       }
       diff.apply(lines, s)
@@ -241,7 +241,7 @@ describe('diff', function()
       s:add({ description = 'Task', recur = 'daily' })
       s:save()
       local lines = {
-        '## Todo',
+        '# Todo',
         '/1/- [ ] Task',
       }
       diff.apply(lines, s)
@@ -252,7 +252,7 @@ describe('diff', function()
 
     it('parses rec: with completion mode prefix', function()
       local lines = {
-        '## Inbox',
+        '# Inbox',
         '- [ ] Water plants rec:!weekly',
       }
       diff.apply(lines, s)
@@ -266,7 +266,7 @@ describe('diff', function()
       s:add({ description = 'Task name', priority = 1 })
       s:save()
       local lines = {
-        '## Inbox',
+        '# Inbox',
         '/1/- [ ] Task name',
       }
       diff.apply(lines, s)