fix(security): replace startswith bypass with is_relative_to in inline_executor

The _validate_paths() method used str(resolved).startswith(str(sandbox_resolved))
to check sandbox containment, which can be bypassed by sibling directories
with name prefixes matching the sandbox root (e.g., /tmp/sandboxmalicious/
would pass a startswith('/tmp/sandbox') check).

Replaced the vulnerable string comparison with Path.is_relative_to() which
performs proper semantic path containment checking.

Closes #7478

src/cleveragents/skills/inline_executor.py

@@ -263,12 +263,12 @@ class InlineToolExecutor:
                 try:
                     resolved = Path(value).resolve()
                     sandbox_resolved = sandbox_path.resolve()
-                    if not str(resolved).startswith(str(sandbox_resolved)):
+                    if not resolved.is_relative_to(sandbox_resolved):
                         return (
                             f"Path '{value}' for key '{key}' escapes sandbox "
                             f"root '{sandbox_path}'"
                         )
-                except (OSError, ValueError):
+                except OSError:
                     return f"Invalid path '{value}' for key '{key}'"
         return None