Fix: include off-hand items when discarding (fixes #553)

[shekinahfire77]

This PR fixes issue #553 where items held in the off-hand were not found by skills.discard and therefore could not be tossed.

Changes:

• src/agent/library/skills.js: discard() now searches bot.inventory.slots as a fallback (this includes off-hand slots) and uses the existing bot.toss() to remove items.
• tests/test_discard_offhand.js: small unit-style test that stubs a bot with an off-hand-like slot and verifies discard calls toss.

Why:

• bot.inventory.items() may not include off-hand/armor slots in some runtimes; scanning inventory.slots ensures items in off-hand are considered.

Notes:

• The test is lightweight and does not require a Minecraft server.
• I kept changes minimal to avoid repo-wide style edits.

Please let me know if you'd like the test excluded from the PR; otherwise CI/maintainers can review and merge.

[uukelele-scratch]

#618 attempts to achieve a much simpler solution

[uukelele-scratch]

seems to include #659 inside here as well