Review follow-ups L + I: drop dead openParty, unify the menu-index parser

L: openParty had no production callers (invite path uses joinParty -> seatLeader,
which seats the leader the same way but reads the owner off the expedition row).
Removed it; the tests now seat the leader through a seatLeaderFixture that wraps
seatLeader in a transaction.

I: promoted parseTemperIndex to parseMenuIndex and routed resolveMagicEquipReply
and handleMasterworkEquipReply through it, replacing two hand-inlined copies of
the same digit parser. Behaviour-preserving (the parsed flag was redundant with
the idx<0 guard); the retry-on-bad-parse disagreement is left as-is since this
is a pure dedup.

Full plugin suite green.

Claude-Session: https://claude.ai/code/session_017mEwUmmS7aQTP2NQXj6rUa

internal/plugin/adventure_masterwork.go

@@ -435,18 +435,8 @@ func (p *AdventurePlugin) handleMasterworkEquipReply(ctx MessageContext, interac
 		return p.SendDM(ctx.Sender, "Equip cancelled.")
 	}
 
-	// Parse number
-	idx := 0
-	for _, c := range reply {
-		if c >= '0' && c <= '9' {
-			idx = idx*10 + int(c-'0')
-		} else {
-			break
-		}
-	}
-	idx-- // 1-indexed to 0-indexed
-
-	if idx < 0 || idx >= len(data.Items) {
+	idx, ok := parseMenuIndex(reply, len(data.Items))
+	if !ok {
 		return p.SendDM(ctx.Sender, "Invalid selection. Reply with a number from the list, or \"cancel\".")
 	}