Remove GlobalCooldown dead code

[Copilot]

GlobalCooldown became dead code after the guess command was switched to per-user CooldownManager. Nothing in production references it anymore.

Changes

• bot/cooldown.py — deleted GlobalCooldown class
• tests/test_cooldown.py — removed TestGlobalCooldownInit, TestGlobalCooldownIsOnCooldown, TestGlobalCooldownSetDuration, and the now-unused GlobalCooldown import

bot/cooldown.py reaches 100% test coverage after removal.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Cooldown seems not be by user but for all</issue_title>
<issue_description></issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Cooldown seems not be by user but for all #33

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