Fix: enforce plain text output in all AI prompts (no markdown)

Add PLAIN TEXT ONLY rule to CORE_RULES so no prompt returns asterisks,
pound signs, or other markdown symbols — output is copy-ready as-is.

src/utils/prompts.js

@@ -9,6 +9,9 @@ CRITICAL RULES FOR ALL OUTPUTS:
 - Use professional medical language
 - Be concise but thorough
 - Output ONLY the requested text, no meta-commentary
+- PLAIN TEXT ONLY: do NOT use markdown formatting — no asterisks (*), no pound signs (#), no underscores (_), no backticks, no bold/italic markers
+- Use plain text section labels followed by a colon (e.g. "Assessment:") on their own line for headers
+- Use plain numbered lists (1. 2. 3.) or plain line breaks for structure, never markdown bullets
 `;
 
 const PROMPTS = {