feat(files-mcp): add Pydantic models and pure fs_ops layer

Adds files_mcp/models.py (FileEntry, OperationResult) and
files_mcp/core/fs_ops.py with 9 pure file operations on already-resolved
Path objects: create/read/update/delete/list_dir/stat/search/move/copy.

All writes go through tempfile.mkstemp + os.replace for atomicity;
read enforces utf-8 + 1MB cap; create/update enforce strict presence
semantics (create: absent or overwrite; update: present). 39 unit tests
in tests/unit/test_fs_ops.py.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

files_mcp/models.py

@@ -0,0 +1,23 @@
+# coding=utf-8
+"""
+@Time   :2026/6/30
+@Author :tao.chen
+"""
+from typing import Literal
+
+from pydantic import BaseModel
+
+
+class FileEntry(BaseModel):
+    path: str            # absolute, resolved, inside sandbox
+    name: str
+    type: Literal["file", "directory", "symlink", "other"]
+    size: int            # bytes; 0 for directories
+    mtime: float         # unix seconds
+    mode: int            # st_mode (octal, e.g. 0o644)
+
+
+class OperationResult(BaseModel):
+    path: str
+    status: str          # "CREATED" | "UPDATED" | "DELETED" | "MOVED" | "COPIED"
+    size: int | None     # None for delete/move