#167: Fix all 7 clippy warnings — annotated SeamMetadata dead fields,
removed unused should_cycle calls, collapsed nested ifs, fixed
useless_format and nonminimal_bool.
#168: Wire TokenUsage mailbox drain to subagent_cost accumulator.
handle_subagent_mailbox now intercepts TokenUsage before routing to
cards, computes cost via calculate_turn_cost, and increments
app.subagent_cost in real time. Footer reflects live sub-agent spend.
Restored ArchivedContext variant to HistoryCell (corrupted by prior
apply_patch). Version bump to 0.7.2.
Refs: #166, #167, #168
Hunter Bown
35db361a87