feat(tools): add bounded file-read windows

[cniska]

Motivation

Full-file reads make common search-read-edit flows burn unnecessary tokens and preserve too much context across turns.

Summary

• add bounded file-read windows around matched lines
• keep full-file reads available when broad context is needed
• cache bounded read windows separately while invalidating by path
• cover bounded reads, cache behavior, and tool guidance in tests

Fixes #262