Add mute reason support

[11k]

Summary

• The mute command now accepts an optional reason after the username (e.g. !mute 10m user spamming links)
• Updated the mute regex to match the ban command's pattern, supporting perm duration and reason capture group
• Reason is appended to the mute punishment string when provided: Muting: {user} for {duration}. Reason: {reason}

Test plan

• Added new mute test file with 4 test cases covering: duration only, duration + reason, default duration, default duration + reason
• Verify existing ban tests still pass (npm test)
• Manual test: !mute 10m user reason text includes reason in punishment
• Manual test: !mute 10m user still works without reason