Add co-op dungeon system (party runs, voting, betting, gifts, items)

Multi-day party runs with funding decisions, TwinBee-narrated floor events,
spectator parimutuel betting, basket/mimic gift system, and weighted-roll
item distribution including masterwork drops at T4-T5.

Schema (5 tables, 2 fewer than spec by computing helpfulness on-the-fly and
skipping the loot-pending state):
- coop_dungeon_runs / _members (daily funding as JSON column)
- coop_dungeon_events (votes as JSON, used to derive TwinBee helpfulness)
- coop_dungeon_bets / _gifts

Mechanics:
- 2-4 player parties, 24h invite window, locks consume one combat action
- Per-floor success roll: base + sum(funding) + level/pet bonuses + event
  vote modifier + active gift modifiers, clamped to 5..95
- Funding tiers (none/min/std/agg/all_in) with liability cap at +8% for
  under-leveled players
- TwinBee narrates events from existing flavor pool; 20 authored events with
  per-option modifiers and embedded recommendation
- Parimutuel betting with 10% rake; odds line shown on lock/daily/status posts
- Gift modifiers symmetric at 50/50 sender mix so no dominant strategy
- Item drops on success via weight-by-contribution roll; T4 25% / T5 100%
  masterwork chance (random pick from existing T4/T5 defs)

Balance via Monte Carlo (coop_dungeon_balance_test.go):
- All tiers exceed 1.5x solo daily income at average party profile
- Optimal funding strategy walks up tiers correctly (Min/Std/Std/Mixed/Agg)
- All-In stays -EV at every tier (boost lever, not optimal play)

Tests: parsing, liability cap, JSON roundtrips, vote tally with leader
tiebreak, event meta consistency, parimutuel payouts, gift EV symmetry,
weighted-roll distribution (Monte Carlo), masterwork tier gates.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/plugin/adventure.go

@@ -141,6 +141,7 @@ func (p *AdventurePlugin) Init() error {
 	go p.robbieTicker()
 	go p.hospitalNudgeTicker()
 	go p.mortgageTicker()
+	go p.coopTicker()
 
 	// Auto-cashout any arena runs left in 'awaiting' from a prior restart
 	p.arenaCleanupStaleRuns()
@@ -185,6 +186,11 @@ func (p *AdventurePlugin) OnMessage(ctx MessageContext) error {
 		return p.handleHospitalCmd(ctx)
 	}
 
+	// 1c. Co-op dungeon commands (work in rooms and DMs)
+	if p.IsCommand(ctx.Body, "coop") {
+		return p.handleCoopCmd(ctx)
+	}
+
 	// 2. Check if this is a DM reply from a registered player
 	p.mu.Lock()
 	playerID, isDM := p.dmToPlayer[ctx.RoomID]
@@ -281,6 +287,7 @@ const advHelpText = `**Adventure Commands**
 ` + "`!adventure blacksmith`" + ` — Visit the blacksmith (view repair costs)
 ` + "`!adventure repair all`" + ` — Repair all damaged equipment
 ` + "`!adventure repair <slot>`" + ` — Repair a specific slot
+` + "`!coop`" + ` — Co-op dungeons (multi-day party runs). See ` + "`!coop help`" + `.
 ` + "`!hospital`" + ` — Visit St. Guildmore's Memorial Hospital (same-day revival when dead)
 ` + "`!thom`" + ` — Visit Thom Krooke (housing and loans)
 ` + "`!adventure help`" + ` — This message