core/tools: edit results preview the old content

Wrong-target edits (the test_run_aside pass clobber, the edit_replace
mangling) were silent: the success message reported counts only, so the
model had no signal it replaced the wrong thing until later.

edit_lineno now reports the first removed line ("first: 'pass'") and
edit_replace reports the matched pattern ('old' -> 'new'), giving the
model an immediate self-correction signal.

tests/test_tools/test_edit_lineno.py

@@ -19,6 +19,7 @@ def test_edit_lineno_replace_middle(workspace: object) -> None:
     _ = _read_lineno("a.txt")
     out = call_sync(edit_lineno, "a.txt", 2, 3, "TWO\nTHREE\n")
     assert "replaced lines 2-3" in out
+    assert "first: 'two'" in out  # old-content excerpt for self-correction
     assert call_sync(read_file, "a.txt") == "L1-4\none\nTWO\nTHREE\nfour\n"
 
 

tests/test_tools/test_file.py

@@ -74,6 +74,7 @@ def test_edit_replace_first_only(workspace: object) -> None:
     assert call_sync(read_file, "t.txt") == "L1-1\nbb aa"
     assert "1 of 2" in result or "1 of 2 matches" in result
     assert "remain" in result
+    assert "'aa' → 'bb'" in result  # old→new excerpt for self-correction
 
 
 def test_edit_replace_max_replaces(workspace: object) -> None: