fix: handle equipped items in inventory actions

[tylergibbs1]

summary

findInventoryItem only searches regular player inventory slots, so discard, giveToPlayer, and putInChest treated armor and offhand items as missing.

This change:

• checks armor and offhand slots when regular inventory has no match
• discards from the exact matched slot, including partial equipped stacks
• moves equipped items back into regular inventory before a chest deposit
• stops safely when the inventory has no room instead of dropping equipped gear

Closes #800

testing

• node --check src/agent/library/skills.js
• exercised the exported discard and putInChest functions with mocked offhand inventory
• verified a partial offhand discard and a complete offhand chest deposit
• verified chest deposit fails safely when normal inventory is full
• git diff --check

I did not run a live Minecraft session or ESLint because this sparse checkout does not have the project dependencies installed.