fix: preserve literal dollars in filesystem edits

[he-yufeng]

Summary

edit_file currently passes newText as the string replacement argument to String.prototype.replace. That makes literal $ sequences behave like replacement patterns, so values such as $$100 and $& are silently rewritten before the file is saved.

This switches the exact-match path to the callback form of replace, which preserves the existing first-match semantics while treating the replacement as literal text.

Changes

• Use modifiedContent.replace(normalizedOld, () => normalizedNew) for exact text edits.
• Add a regression test covering $$ and $& in replacement text.

To verify

• npm exec -w src/filesystem -- vitest run __tests__/lib.test.ts --testNamePattern "dollar signs"
• npm exec -w src/filesystem -- vitest run __tests__/lib.test.ts
• npm run build -w src/filesystem
• git diff --check origin/main...HEAD

Fixes #4157