Adventure manager: focus/archive/edit tooling for multi-adventure campaigns

[lappat]

For campaigns that span multiple discrete adventures/arcs (not just one
long continuous story), it'd help to have GM tooling to mark which
adventure is currently "in focus" (affects which mission-scoped state
keys are active — see the campagne_/m__ key-naming split in
docs/ARCHITECTURE.md), archive a completed adventure, and view/edit the
adventure manifest directly rather than by hand-editing JSON.