docs: add subdirectory CLAUDE.md files for key domain modules

Provide non-obvious business rules and constraints for economy,
inventory, quest, moderation, trade, and trivia modules to reduce
context-gathering overhead for AI tools.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

shared/modules/inventory/CLAUDE.md

@@ -0,0 +1,11 @@
+# Inventory Module
+
+- Inventory has two hard limits from config: **max slots** (distinct item types) and **max stack size** (quantity per item). Both are enforced in `addItem` and will throw `UserError` if exceeded.
+- When quantity reaches 0, the inventory row is **deleted** (not kept with quantity 0). This means slot count = row count.
+- `buyItem` delegates balance deduction to `economyService.modifyUserBalance` within the same transaction to ensure atomicity. Never deduct balance directly when purchasing.
+- Item usage is driven by a JSON `usageData` field on the item record. Items without `usageData.effects` cannot be used. The `consume` flag in `usageData` controls whether the item is removed after use.
+- **Effect system**: effects are validated at runtime via Zod (`EffectPayloadSchema`) before execution. The registry maps effect type strings to handler functions. Adding a new effect type requires: (1) add to `EffectType` enum in constants, (2) add Zod schema variant in `effect.types.ts`, (3) add handler in `effect.handlers.ts`, (4) register in `effect.registry.ts`.
+- `XP_BOOST` and `TEMP_ROLE` effects use `userTimers` with upsert -- activating while already active **replaces** the timer (does not stack or extend).
+- `TEMP_ROLE` only records the timer in DB; actual Discord role assignment must happen in the bot command layer.
+- `LOOTBOX` effect uses weighted random selection. Weights are relative, not percentages. A `NOTHING` loot type is valid and intentional.
+- The `getAutocompleteItems` method filters to only show items that have usable effects, so non-usable items won't appear in the `/use` autocomplete.