Document pickle loader trust requirements

[skrawcz]

Clarifies Apache Hamilton's security model and documents the trust requirements for Python pickle deserialization.

Changes

• document that dataflows, extensions, and adapters execute with the host process privileges
• warn that the pickle loader must only read trusted files
• recommend non-executable interchange formats across trust boundaries

How I tested this

• git diff --check
• .venv/bin/ruff check hamilton/io/default_data_loaders.py
• .venv/bin/ruff format --check hamilton/io/default_data_loaders.py

Notes

This is documentation hardening only; it does not change runtime behavior.