Filter simulation personas to agent-eligible entities

[samrusani]

Summary

Fixes #759 by filtering graph entities before they are promoted into simulation personas.

The simulation prepare flow now builds its roster from agent-eligible person entities only, so non-person graph nodes such as project names, companies, topics, or sentence fragments do not become interviewable agents. The same filtered roster is passed to both profile generation and simulation config generation.

The standalone profile generation endpoint also applies the persona filter by default, with an explicit allow_group_accounts opt-in for callers that intentionally want institution/group accounts.

Root Cause

ZepEntityReader.filter_defined_entities() only removed default-only graph nodes. Any node with a custom label could then flow into generate_profiles_from_entities() and SimulationConfigGenerator.generate_config(), where it was treated as an agent candidate.

That allowed entities like MiroFish or market event to become simulated participants even when the simulation requirement expected individual people.

Validation

cd backend
uv run pytest tests

Result: 133 passed.

[samrusani]

Closing this to reduce my stale contributor backlog. This draft never developed a maintainer review path and would need a fresh scope check against current main before it could be advanced safely. If a maintainer wants this continued, I’m happy for the PR to be reopened.