4-tier permission model: user < trusted < oper < admin.
Commands specify a required tier via tier= parameter.
Backward compatible: admin=True maps to tier="admin".
- TIERS constant and Handler.tier field in plugin.py
- _get_tier() method in bot.py with pattern matching
- _is_admin() preserved as thin wrapper
- operators/trusted config lists in config.py
- whoami shows tier, admins shows all configured tiers
- 32 test cases in test_acl.py
