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).
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