Enable `E501` at the shared 69-column source limit so `ruff`
surfaces the repository's existing line-length debt.
Keep the debt visible rather than rewriting legacy source, and
record the incremental checker plan for making changed-line
enforcement landable.
Prompt-IO: ai/prompt-io/opencode/20260825T181102Z_3690e43a_prompt_io.md
(this patch was generated in some part by `opencode` using `gpt-5.6-sol` (`openai`))
Gud Boi