fix: remove /start command and handle @botname suffix

Remove /start command as name doesn't match functionality, and fix
command parsing to handle @botname suffix for group chat compatibility.

Changes:
- Remove handle_start function and registration
- Remove /start from help command list
- Remove test_start_command_content test
- Strip @botname suffix from commands (e.g., /help@mybot → help)

Rationale:
- /start command name implies bot initialization, but it was just
  showing help text (duplicate of /help)
- Better to have one clear /help command
- @botname suffix handling needed for group chats

Test:
- 27 tests pass (1 removed, 1 added for @botname handling)
- All existing functionality preserved

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/notifications/telegram_client.py

@@ -492,7 +492,8 @@ class TelegramCommandHandler:
             if not command_parts:
                 return
 
-            command_name = command_parts[0]
+            # Remove @botname suffix if present (for group chats)
+            command_name = command_parts[0].split("@")[0]
 
             # Execute handler
             handler = self._commands.get(command_name)