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/sync/gcal.lua

@@ -177,6 +177,7 @@ function M.push()
         and (
           task.status == 'done'
           or task.status == 'deleted'
+          or task.status == 'cancelled'
           or (task.status == 'pending' and not task.due)
         )