fix(deriver): update prompt to prevent example leakage (#1028)

* fix(deriver): delimit examples and forbid example leakage

Wrap the minimal deriver EXAMPLES block in <examples> tags and add an explicit negative instruction: the examples are fabricated format illustrations only, and every conclusion must be grounded in the <messages> block. Soft mitigation for example-content leakage; real fixes (sources_indices + SFT / structural enforcement) are follow-ups.

* test(deriver): drop low-value examples-delimiter unit test

src/deriver/prompts.py

@@ -71,10 +71,14 @@ RULES:
 - Extract ALL observations from the target peer's messages, using others as context.
 - Contextualize each observation sufficiently (e.g. "Ann is nervous about the job interview at the pharmacy" not just "Ann is nervous")
 
+<examples>
+These examples are fabricated illustrations of the output format. Never emit a conclusion for which content comes from these examples. Every conclusion must be supported by the <messages> block only.
+
 EXAMPLES (using `alice` as the target peer id):
 - EXPLICIT: "I just turned 25" → "alice is 25 years old"
 - EXPLICIT: "I took my dog for a walk in NYC" → "alice has a dog", "alice walked her dog in NYC"
 - EXPLICIT: "I've lived in NYC for six years" → "alice lives in NYC", "alice has lived in NYC for six years"
+</examples>
 
 {custom_instructions_section}