Blocks swapping the gh-aw shim's MultiEdit tool onto the harness. Context: #265 (shim parity is FileSystem + Shell only and does not cover this).

FileSystemToolset.edit_file does one replacement and requires the match to be unique. MultiEdit applies an ordered list of edits to a single file atomically: if any old_string is missing, nothing is written.

Requirement: a method, e.g.

async def multi_edit(self, path: str, edits: list[EditOp], *, expected_hash: str | None = None) -> str

where EditOp is {old_string: str, new_string: str, replace_all: bool = False}, with:

• edits applied in order, each to the result of the previous one
• each edit replaces the first occurrence, or every occurrence when replace_all
• all-or-nothing: if any old_string is not found, the file is left untouched and the call raises a recoverable ModelRetry
• same containment and expected_hash semantics as edit_file

The one semantic difference from edit_file: per-edit matching is first-occurrence, not unique-occurrence.