feat: add cancelled task status with configurable state chars (#158)

Problem: the task lifecycle only has `pending`, `wip`, `blocked`, and
`done`. There is no way to mark a task as abandoned. Additionally,
state characters (`>`, `=`) are hardcoded rather than reading from
`config.icons`, so customizing them has no effect on rendering or
parsing.

Solution: add a `cancelled` status with default state char `c`, `g/`
keymap, `PendingCancelled` highlight, filter predicate, and archive
support. Unify state chars by making `state_char()`, `parse_buffer()`,
and `infer_status()` read from `config.icons`. Change defaults to
mnemonic chars: `w` (wip), `b` (blocked), `c` (cancelled).

lua/pending/config.lua

@@ -11,6 +11,7 @@
 ---@field priority string
 ---@field wip string
 ---@field blocked string
+---@field cancelled string
 ---@field due string
 ---@field recur string
 ---@field category string
@@ -80,6 +81,7 @@
 ---@field blocked? string|false
 ---@field priority_up_visual? string|false
 ---@field priority_down_visual? string|false
+---@field cancelled? string|false
 
 ---@class pending.CategoryViewConfig
 ---@field order? string[]
@@ -160,6 +162,7 @@ local defaults = {
     move_up = 'K',
     wip = 'gw',
     blocked = 'gb',
+    cancelled = 'g/',
     priority_up = '<C-a>',
     priority_down = '<C-x>',
     priority_up_visual = 'g<C-a>',
@@ -190,8 +193,9 @@ local defaults = {
     pending = ' ',
     done = 'x',
     priority = '!',
-    wip = '>',
-    blocked = '=',
+    wip = 'w',
+    blocked = 'b',
+    cancelled = 'c',
     due = '.',
     recur = '~',
     category = '#',