feat: add granular ACL tiers (trusted/oper/admin)

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

tests/test_integration.py

@@ -274,7 +274,7 @@ class TestAdmin:
 
         replies = h.sent_privmsgs("#test")
         assert not any("Permission denied" in r for r in replies)
-        assert any("Patterns:" in r for r in replies)
+        assert any("Admin:" in r for r in replies)
 
     def test_oper_detection(self):
         """IRC operator detected via WHO reply can use admin commands."""
@@ -290,7 +290,7 @@ class TestAdmin:
 
         replies = h.sent_privmsgs("#test")
         assert not any("Permission denied" in r for r in replies)
-        assert any("Opers:" in r for r in replies)
+        assert any("IRCOPs:" in r for r in replies)
 
     def test_oper_detection_on_join(self):
         """User joining a channel triggers debounced WHO for oper detection."""