Add text-based symptom priors

[justfarbod]

The symptoms are defined in a structured way in the YAML case file, using PHQ-9-style symptom dimensions and severity values. During patient prior construction, the symptom data is formatted into a readable symptom profile and included in the patient system prompt.

[justfarbod]

run_symptom_experiments.py is a batch runner for automated simulation experiments.

It uses the empty_and_invisible case as a fixed base case, runs 5 baseline sessions with no symptom field, and then runs 5 sessions per symptom where one symptom is set to "nearly every day" and the others to "not at all".

The script generates temporary case files, calls the existing run.py workflow, and relies on the current session-saving logic to store outputs under data/sessions/.