fix(list): section heading follows the status filter (#592) (#596)

'coi list --stopped' titled its output 'Active Containers:' — the filter
worked, the label lied. The text heading now names the filtered state
(Stopped/Running/Frozen/... Containers:), the underline matches its
length, and unfiltered output keeps the historical heading. JSON is
unchanged ('active_containers' stays, for script compatibility).

e2e: the mixed-state filter test asserts the retitled headings and that
the old heading is gone under a filter. Unit test covers the mapping.

Closes #592.