fix: append_file after_line=-1 inserts at wrong position in prepend mode

[four-bytes-robby]

Bug

Found by cubic-dev-ai on PR #15. In src/tools/append-file.ts, when mode=prepend and after_line=-1, the content is inserted at the wrong position (beginning or after last content line) instead of before the last line.

Expected

after_line=-1 should mean "insert before the last line" (i.e., second-to-last position), consistent with negative array indexing semantics.

Fix

Adjust the line-splitting logic so -1 correctly maps to lines.length - 2 (insert before the last element).