fix(sdk)!: disambiguate read_file line gutters

[Mason Daugherty (mdrxy)]

Warning

This is a breaking change for callers that parse raw read_file output. The line-number gutter no longer uses cat -n-style padding and a tab separator; parsers should expect line markers followed by two spaces before the source content.

read_file returns file contents with line numbers so agents can reference and edit specific lines. Before this PR, those line numbers were rendered like cat -n: a fixed-width line-number gutter followed by a tab.

Before:

     1	def build_config():
     2		return {"enabled": True}

That format is hard to interpret when the source line itself starts with a tab. In the example above, line 2 has two tabs in a row: one tab from the read_file gutter and one tab from the source indentation. A model can mistake both tabs for source content, then send an over-indented old_string to edit_file.

This PR changes the SDK formatter so the line marker is followed by two spaces before the original source content.

After:

1  def build_config():
2  	return {"enabled": True}

Now the gutter is visually and textually distinct from the file contents: the first two spaces separate the line marker from the source, and any tabs after that are actual source indentation.

Long-line continuation markers still work the same way. They now use the same two-space separator:

  2  first wrapped chunk
2.1  second wrapped chunk
2.2  third wrapped chunk

The file contents, line numbers, pagination behavior, and continuation-marker semantics are unchanged. Only the raw text formatting of the line-number gutter changes.

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Title scope(s): sdk

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