fix(runtime): write edit tool replacements literally

[kevin9327]

Summary

When an agent uses the built-in edit tool without replace_all, any $$, $&, $` or $' in new_string is rewritten before it reaches the file. The tool still reports Replaced 1 occurrence, but the file contains different text than the model asked for. Common cases:

• $$HOME or $$(pwd) in a Makefile becomes $HOME / $(pwd)
• $$ (shell PID) or a $$ … $$ math block in Markdown/LaTeX loses a dollar sign
• $& is replaced by the matched old_string, and $' / $` by the rest of the file after / before the match

The cause is that content.replace(oldString, newString) still applies replacement patterns even when the search value is a string. The replace_all path uses split/join and already writes new_string verbatim, so the same edit gives different files depending on that flag. Passing a replacer function (() => newString) makes the single-match path literal too. Matching, uniqueness checks and the result message are unchanged.

Validation

bun test packages/runtime/tests/tools/built-in/fs.test.ts -t "edit writes"

Before the fix (new test added, source unchanged):

packages\runtime\tests\tools\built-in\fs.test.ts:
52 |     await edit(allPath, "echo OLD", newString, true);
53 |     expect(await fs.readFile(allPath, "utf8")).toBe(`run:\n\t${newString}\n`);
54 | 
55 |     await write(singlePath, "run:\n\techo OLD\n");
56 |     await edit(singlePath, "echo OLD", newString);
57 |     expect(await fs.readFile(singlePath, "utf8")).toBe(
                                                       ^
error: expect(received).toBe(expected)

  "run:
- 	echo $$HOME $& $' $`
+ 	echo $HOME echo OLD 
+  run:
+ 	
  "

- Expected  - 1
+ Received  + 3

      at <anonymous> (packages\runtime\tests\tools\built-in\fs.test.ts:57:51)
(fail) filesystem built-in paths > edit writes $ sequences in new_string literally [12.93ms]

 0 pass
 34 filtered out
 1 fail

After the fix, this test passes.

• The replace_all: true half of the test (line 53) passes both before and after, as do the existing write and edit … tests. This shows only the single-match path's output changes.
• Whole file, bun test packages/runtime/tests/tools/built-in/fs.test.ts: 32 pass / 2 fail on main, and 33 pass / 2 fail on this branch. The same two tests fail on main: read, traversal, and search tools expand home paths and grep caps multibyte matching output. Both need ripgrep (rg), which is not installed locally; CI installs it.
• ESLint (--max-warnings 0) on both touched files: clean on main and on this branch.
• tsc --noEmit -p packages/runtime/tsconfig.json: clean on main and on this branch.
• Prettier: both files already have unformatted lines on main (9 and 62); this change adds none.

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