tools/file: add with_lineno option to read_file

When true, prefix each returned line with absolute 1-based line numbers
(N|…) for edit_lineno-friendly reads; works with offset/limit.

src/plyngent/tools/file/read.py

@@ -3,12 +3,33 @@ from __future__ import annotations
 from plyngent.agent import tool
 from plyngent.tools.workspace import resolve_path
 
+_LINENO_WIDTH = 6
+
+
+def _format_with_lineno(lines: list[str], *, start_lineno: int) -> str:
+    """Prefix each line with a 1-based absolute line number (``edit_lineno`` style)."""
+    out: list[str] = []
+    for index, line in enumerate(lines):
+        lineno = start_lineno + index
+        # Strip keepends for the body; re-add a single newline after the prefix.
+        body = line.rstrip("\r\n")
+        out.append(f"{lineno:>{_LINENO_WIDTH}}|{body}\n")
+    return "".join(out)
+
 
 @tool
-def read_file(path: str, *, offset: int = 0, limit: int | None = None) -> str:
+def read_file(
+    path: str,
+    *,
+    offset: int = 0,
+    limit: int | None = None,
+    with_lineno: bool = False,
+) -> str:
     """Read a text file under the workspace.
 
     ``offset`` is 0-based line start; ``limit`` is max lines (None = rest of file).
+    When ``with_lineno`` is true, each line is prefixed with its 1-based file line
+    number (``     N|…``), matching ``edit_lineno`` numbering.
     """
     target = resolve_path(path)
     if not target.is_file():
@@ -21,4 +42,7 @@ def read_file(path: str, *, offset: int = 0, limit: int | None = None) -> str:
     end = len(lines) if limit is None else min(len(lines), start + limit)
     if start >= len(lines):
         return ""
-    return "".join(lines[start:end])
+    slice_lines = lines[start:end]
+    if with_lineno:
+        return _format_with_lineno(slice_lines, start_lineno=start + 1)
+    return "".join(slice_lines)

tests/test_tools/test_file.py

@@ -35,6 +35,18 @@ def test_read_offset_limit(workspace: object) -> None:
     assert call_sync(read_file, "lines.txt", offset=1, limit=2) == "b\nc\n"
 
 
+def test_read_with_lineno(workspace: object) -> None:
+    del workspace
+    _ = call_sync(write_file, "num.txt", "a\nb\nc\n")
+    out = call_sync(read_file, "num.txt", with_lineno=True)
+    assert "     1|a\n" in out
+    assert "     2|b\n" in out
+    assert "     3|c\n" in out
+    # offset is 0-based; line numbers stay absolute 1-based file lines
+    mid = call_sync(read_file, "num.txt", offset=1, limit=1, with_lineno=True)
+    assert mid == "     2|b\n"
+
+
 def test_listdir_missing(workspace: object) -> None:
     del workspace
     assert "error" in call_sync(listdir, "nope")