docs: wire mean_field_summary + diagnostics artifacts into the EP walkthrough

[Jammy2211]

Overview

Follow-up from PyAutoFit#1335 (merged #1349): end scripts/features/expectation_propagation.py (merged via #82) with the built-in diagnostics — a mean_field_summary() call and pointers at the emitted ep_history.csv / mean_field_evolution.png / graph_factors.png artifacts — so the walkthrough demonstrates the Phase-4 tooling instead of stopping at the raw mean field.

Prompt: PyAutoMind/docs/autofit_workspace/ep_walkthrough_mean_field_summary.md. Suggested branch feature/ep-walkthrough-diagnostics.
Autonomy: safe (docs, easy; --auto chain, human directed 2026-07-10).

[Jammy2211]

Shipped at PR-open — #85 (pending-release). __Diagnostics__ section wired: mean_field_summary() table + live-artifact tour + check_sigma_collapse pointer; new calls exercised against the installed library. Merge stays human.

[Jammy2211]

Merged — 831b08ba via #85. Closing.