INPUT_HIGHLIGHT_PATTERN

Pattern for highlighting @mentions and /commands in rendered user messages.

Matches either:

• Slash commands at the start of the string (e.g., /help)
• @file mentions anywhere in the text (e.g., @README.md)

Note: The ^ anchor matches start of string, not start of line. The consumer in UserMessage.compose() additionally checks start == 0 before styling slash commands, so a / mid-string is not highlighted.