review: tighten scope and add regression tests

Follow-ups from review of #3194:

- ci.yml: drop unconditional --ignore=tests/channels/test_matrix_channel.py.
  That test file already calls pytest.importorskip("nio") at module top, so
  it self-skips on Windows (where nio isn't installed) without also hiding
  62 tests from Linux CI.

- filesystem.py: hoist `import os` to the module top and drop the duplicate
  inline import in ReadFileTool.execute. Document the CRLF->LF normalization
  as intentional (primarily a Windows UX fix so downstream StrReplace/Grep
  match consistently regardless of where the file was written).

- test_read_enhancements.py: lock down two new behaviors
  * TestFileStateHashFallback: check_read warns when content changes but
    mtime is unchanged (coarse-mtime filesystems on Windows).
  * TestReadFileLineEndingNormalization: ReadFileTool strips CRLF and
    preserves LF-only files untouched.

- test_tool_validation.py: restore list2cmdline/shlex.quote in
  test_exec_head_tail_truncation. The temp_path-based form was correct,
  but dropping the quoting broke on any Windows path containing spaces
  (e.g. C:\Users\John Doe\...). CI runners happen not to have spaces so
  this slipped through.

Tests: 1993 passed locally.
Made-with: Cursor

tests/tools/test_tool_validation.py

@@ -1,3 +1,4 @@
+import shlex
 import subprocess
 import sys
 from typing import Any
@@ -547,13 +548,20 @@ async def test_exec_always_returns_exit_code() -> None:
 async def test_exec_head_tail_truncation(tmp_path) -> None:
     """Long output should preserve both head and tail."""
     tool = ExecTool()
-    # Generate output that exceeds _MAX_OUTPUT (10_000 chars)
-    # Use a temp script file to avoid Windows command line quote parsing issues
+    # Generate output that exceeds _MAX_OUTPUT (10_000 chars).
+    # Use a temp script file so the output-generating logic lives in a file
+    # (Windows cmd.exe has finicky rules for quoting `-c` payloads with
+    # embedded newlines). ExecTool runs via create_subprocess_shell, so we
+    # must quote *both* the interpreter path and the script path — tmp_path
+    # on some CI runners and on many local Windows installs contains spaces
+    # (e.g. C:\Users\John Doe\AppData\...) which would otherwise break the
+    # shell's argv split.
     script_file = tmp_path / "gen_output.py"
     script_file.write_text("print('A' * 6000 + chr(10) + 'B' * 6000)", encoding="utf-8")
-    # On Windows, cmd.exe handles quotes differently. Use the path directly
-    # without additional quotes since the temp path shouldn't have spaces.
-    command = f"{sys.executable} {script_file}"
+    if sys.platform == "win32":
+        command = subprocess.list2cmdline([sys.executable, str(script_file)])
+    else:
+        command = f"{shlex.quote(sys.executable)} {shlex.quote(str(script_file))}"
     result = await tool.execute(command=command)
     assert "chars truncated" in result
     # Head portion should start with As