This commit introduces the `SynthesisAgent` for generating narrative research reports from structured data, addressing the issue of outputting synthesized prose instead of structured metadata. Key changes include:

- Creation of `SynthesisAgent` in `src/agents/synthesis.py` for LLM-based synthesis.
- Addition of synthesis prompts and a few-shot example in `src/prompts/synthesis.py`.
- Modification of the orchestrator to call the new `SynthesisAgent` for report generation.
- Implementation of tests to ensure narrative output structure and reference accuracy.

This enhancement aligns with the project's focus on sexual health research and improves the quality of report generation.

Closes #85, #86.