Add Inventory Module

[Mkalbani]

Description

Create an inventory module that manages items (tools, tokens) a player collects during gameplay.

Acceptance Criteria

• An inventory module exists under src/inventory/mod.rs
• Inventory struct holds a list of Item with id, name, and quantity
• Items can be added, removed, and queried
• Inventory can be serialized for persistence
• Unit tests cover add, remove, overflow, and empty cases

[ayomideadeniran]

I'm a full-stack developer and I'd like to take this issue.
I understand the requirements and will follow the existing project structure, keep the implementation clean, and include proper tests where applicable.

ETA: 1–3 days (depending on scope)
Portfolio: https://knightsdev-portfolio.vercel.app/