Replace the `{ enabled: boolean; maxLines: number; maxBytes: number }`
shape with `Option<{ maxLines: number; maxBytes: number }>` so the absence
of limits is represented by the type system instead of a boolean flag with
dead numeric fields.
- Truncate.limits() now returns Effect<Option<Limits>>.
- Truncate.output() short-circuits on None instead of checking .enabled.
- shell tool gates rolling buffer, disk spill, and tail truncation on
Option.isSome(limits); behavior unchanged when truncation is enabled.
- ShellPrompt.render and helpers accept Option<Limits>; the truncation
guidance line is omitted when None.
- Tests updated to assert Option.isSome / Option.isNone.
93cc5e8dff