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/magic_items_gameplay.go

@@ -579,18 +579,8 @@ func (p *AdventurePlugin) resolveMagicEquipReply(ctx MessageContext, interaction
 		return p.SendDM(ctx.Sender, "Equip cancelled.")
 	}
 
-	idx := 0
-	parsed := false
-	for _, c := range reply {
-		if c >= '0' && c <= '9' {
-			idx = idx*10 + int(c-'0')
-			parsed = true
-		} else {
-			break
-		}
-	}
-	idx-- // 1-indexed → 0-indexed
-	if !parsed || idx < 0 || idx >= len(data.Items) {
+	idx, ok := parseMenuIndex(reply, len(data.Items))
+	if !ok {
 		p.pending.Store(string(ctx.Sender), interaction)
 		return p.SendDM(ctx.Sender, "Invalid selection. Reply with a number from the list, or \"cancel\".")
 	}