read: directory listing returns an empty page for out-of-range offset instead of erroring

Describe the bug

Directory listing in the read tool silently returns an empty page when offset exceeds the number of visible entries (visible.slice(offset - 1, ...) yields [] with truncated: false). Reading a text file with the same out-of-range offset fails with OffsetOutOfRangeError.

The model can't distinguish "this directory really has this few entries" from "offset went past the end", and a truncated page's next can point to a page that is necessarily empty.

Steps to reproduce

{ path: "src", offset: 100 } on a directory with 5 entries → 200 with { entries: [], truncated: false }; the same offset on a text file → Offset 100 is out of range.

Affected code

packages/core/src/tool/read-filesystem.ts — directory listing slice, vs. the text read path's offset validation.

Suggested fix

Fail with OffsetOutOfRangeError for out-of-range directory offsets, consistent with text reads.

Environment

• opencode version: latest dev