Yoink it from https://git.secluded.site/yadr/blob/main/dot_config/private_fish/functions/doc-agent.fish?raw=1 into ~/.config/fish/functions/doc-agent.fish

doc-agent also relies on synu. Refer to https://git.secluded.site/agent-skills/blob/main/invoking-subagents/references/installing-synu.md?raw=1 if it's unavailable.