adventure: tell Pete this build knows what a party seat is

The party list is omitempty, so a solo run and a game box too old to push
seats reach Pete as the same empty slice, and those two want opposite
buttons. Pete's page was reading the empty one as "solo, therefore leader"
and offering a party member the button that throws away everyone's day.

One boolean, set on every sheet a build makes — in town, solo, or seated
with three others. It says something about the sender, never about the
character, so it is set unconditionally: a conditional flag would read as
"this player is solo" and land straight back in the hole it closes. An
older build sends no key at all, which decodes to false, which is the
answer Pete wants when it cannot tell.

Pete's half shipped already and withholds the button until this arrives.

Verified end to end against a local Pete, not just in tests: a solo leader
on a live expedition gets "Call the whole thing off" back, a member still
gets "Turn back alone" and never the abandon, and stripping the flag from
the same push takes the solo button away again.

internal/plugin/pete_roster.go

@@ -144,6 +144,10 @@ func rosterDetail(uid id.UserID, c *DnDCharacter) *peteclient.RosterDetail {
 		ArmorClass: c.ArmorClass,
 		Abilities:  [6]int{c.STR, c.DEX, c.CON, c.INT, c.WIS, c.CHA},
 		Modifiers:  c.Modifiers(),
+		// Unconditional, and this is the whole point of the field: it says this
+		// build knows about party seats, not that this character has any. Making
+		// it conditional would put it straight back in the hole it closes.
+		PartyKnown: true,
 	}
 	if equip, err := loadAdvEquipment(uid); err == nil {
 		for _, slot := range allSlots {