fix(fs_write): preserve CRLF line endings

[voidstackloop]

Fixes #1751.

Summary

• detect the existing file's line ending before str_replace
• translate only the requested old and new strings, preserving unrelated file content
• cover both the agent and chat CLI implementations with CRLF regression tests

Tests

• cargo test -p agent agent::tools::fs_write::tests (10 passed)
• cargo test -p chat_cli cli::chat::tools::fs_write::tests (14 passed)
• cargo clippy --locked -p agent -p chat_cli -- -D warnings
• nightly rustfmt --check on both modified files