fix(security): prevent path traversal bypass via startswith check

`startswith` string comparison allows bypassing directory restrictions.
For example, `/home/user/workspace_evil` passes the check against
`/home/user/workspace` because the string starts with the allowed path.

Replace with `Path.relative_to()` which correctly validates that the
resolved path is actually inside the allowed directory tree.

Fixes #888

nanobot/agent/tools/filesystem.py

@@ -13,8 +13,11 @@ def _resolve_path(path: str, workspace: Path | None = None, allowed_dir: Path |
     if not p.is_absolute() and workspace:
         p = workspace / p
     resolved = p.resolve()
-    if allowed_dir and not str(resolved).startswith(str(allowed_dir.resolve())):
-        raise PermissionError(f"Path {path} is outside allowed directory {allowed_dir}")
+    if allowed_dir:
+        try:
+            resolved.relative_to(allowed_dir.resolve())
+        except ValueError:
+            raise PermissionError(f"Path {path} is outside allowed directory {allowed_dir}")
     return resolved