fix(ui): preserve multiline slash command input

[zhengdaqi]

Summary

• preserve raw slash command input/args when executing commands from the chat composer
• use raw passthrough content for bundled and on-disk skill commands so multiline prompts are not truncated
• detect slash command names across any whitespace separator, including newlines

Verification

• pnpm install --frozen-lockfile
• pnpm --dir ui run build
• reproduced on a clean OpenBMB/PilotDeck deployment: multiline /ceremony-speech-writing input was truncated to the first line before this fix
• verified patched cb6 deployment preserves multiline accepted_input in session history