core/tools: edit_lineno unread error names the read window

The boundary overguard (read offset=50 shows lines 51+, then editing
line 50 fails) forced blind re-reads. The error now reports the read
window ("this turn read lines 51-86") and advises reading with a margin
around the target range; read_file's description says the same.

src/plyngent/tools/file/edit_lineno.py

@@ -111,13 +111,20 @@ async def edit_lineno(path: str, start_line: int, end_line: int, new_content: st
     if err is not None:
         return err
 
-    unread = _unread_lines_in_range(lineno_read_lines(str(target)), start_line, end_line, len(lines))
+    read_lines = lineno_read_lines(str(target))
+    unread = _unread_lines_in_range(read_lines, start_line, end_line, len(lines))
     if unread:
         preview = unread[:_UNREAD_PREVIEW]
         head = ", ".join(str(i) for i in preview)
         if len(unread) > _UNREAD_PREVIEW:
             head += f", … ({len(unread)} total)"
-        return f"error: lines {head} not read; call read_file(path, with_lineno=true) first to see current line numbers"
+        window = ""
+        if read_lines:
+            window = f" (this turn read lines {min(read_lines)}-{max(read_lines)})"
+        return (
+            f"error: lines {head} not read{window}; call read_file(path, with_lineno=true) "
+            "first — read with a small margin around the lines you plan to edit"
+        )
 
     recorded_mtime = lineno_read_mtime(str(target))
     if recorded_mtime is not None:

src/plyngent/tools/file/read.py

@@ -64,6 +64,8 @@ async def read_file(
     (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, and those lines are marked readable for ``edit_lineno`` this turn.
+    Only the displayed lines become editable, so read with a small margin around
+    any range you plan to edit with ``edit_lineno``.
 
     ``max_chars`` caps the returned slice (default: the agent's tool-result cap,
     ``DEFAULT_TOOL_RESULT_MAX_CHARS``); a ``truncate_token`` is appended when cut