edit: newString dollar patterns are expanded, silently corrupting files

Describe the bug

The edit tool passes the model's newString directly as the replacement string to String.prototype.replace/replaceAll. A string replacement interprets $\&, $```, $', $n, and $$` as special patterns.

So an edit whose replacement contains a dollar pattern silently corrupts the file. $\& and $1/shell positional params / sed``awk snippets and JS template literals are extremely common in code, and the preview patch in the tool result does not show it: the tool reports success and the corruption lands on disk with content the model never wrote.

Steps to reproduce

File contains:

log("done");

Edit: oldString: 'log("done");', newString: 'log("$& ok");'

Expected: log("$& ok");
Actual: log("log("done"); ok"); ($& expanded to the matched text)

Affected code

• V2: packages/core/src/tool/edit.ts (replaceAll and replace)
• V1: packages/opencode/src/tool/edit.ts (replaceAll path only)

Fix is to use a function replacer (() => newString) so the replacement is inserted verbatim.

Environment

• opencode version: latest dev
• OS: any