Add chat level perks: XP bonus, shop flavor, rare drop nudge, death pardon (Adventure 2.5 steps 3-6)

- XP bonus: +5% per 10 chat levels (cap +25% at 50+), applies to all
  adventure and arena XP
- Shop flavor: shopkeeper greeting changes at chat levels 10/30/50,
  replacing random flavor for recognized players
- Rare drop nudge: +0.5% per 10 chat levels (cap +2.5%) applied
  additively to treasure and masterwork drop rates
- Death pardon: chat level 20+ gets 33% chance to survive death once
  per 7 days (converts to bad-failure, quiet DM). Does not apply in
  arena. Fires before Sovereign Death's Reprieve check.
- New adventure_chat_perks.go with perk calculation helpers
- LastPardonUsed field + migration on adventure_characters

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/plugin/adventure_chat_perks.go

@@ -0,0 +1,38 @@
+package plugin
+
+// ── Chat Level Perks ────────────────────────────────────────────────────────
+// Passive bonuses based on community chat level. All perks are automatic.
+
+// chatLevelXPBonus returns the fractional XP bonus for the given chat level.
+// +5% per 10 levels, capped at +25% at level 50+.
+func chatLevelXPBonus(chatLevel int) float64 {
+	tier := chatLevel / 10
+	if tier > 5 {
+		tier = 5
+	}
+	return float64(tier) * 0.05
+}
+
+// chatLevelRareBonus returns the additive rare drop rate bonus.
+// +0.5% per 10 levels, capped at +2.5% at level 50+.
+func chatLevelRareBonus(chatLevel int) float64 {
+	tier := chatLevel / 10
+	if tier > 5 {
+		tier = 5
+	}
+	return float64(tier) * 0.005
+}
+
+// shopGreeting returns the shopkeeper's greeting based on chat level.
+func shopGreeting(chatLevel int) string {
+	switch {
+	case chatLevel >= 50:
+		return "I saw you coming from down the street. Your usual is already on the counter. The shop is open."
+	case chatLevel >= 30:
+		return "Back again. I've started keeping your usual in the back. The shop is open."
+	case chatLevel >= 10:
+		return "Ah. You again. The shop is open."
+	default:
+		return "The shop is open. What do you need?"
+	}
+}