These changes help the agent edit files when `<old_text>` matches more
than one location.

First, the agent can specify an optional `<old_text line=XX>` parameter.
When this is provided and multiple matches exist, we use this hint to
identify the best match.

Second, when there is ambiguity in matches, we now return the agent a
more helpful message listing the line numbers of all possible matches.

Together, these changes should reduce the number of misplaced edits and
agent confusion.

I have ensured the LLM Worker works with these prompt changes.


Release Notes:

- Agent: Improved locating edits