Bot command: `mark` - show per-world item receives and hints since last mark

Support a marker start and stats lookup that shows item receive and hint counts applying to the a world context since a marker was placed.

Aggregate items by priority. Aggregate hints by the current "for"/"from" split, plus "outstanding" or "found". Do not store item names or hint details, just counts.

Commands:

• mark - view the current status of a marker only - if one is set, and if so, when; do not report the stats or remove the marker
• mark add - start recording item and hint receive counts
• mark report - view the accumulated stats and remove the marker