fix(reflect): prevent directive leakage on empty banks

[nicoloboschi]

Summary

• When a bank has directives but no memories/facts, the reflect agent echoes directive content verbatim as its answer instead of saying it has no information
• Root cause: the LLM short-circuits (returns text, no tool calls) and parrots the "MANDATORY" directive text from its system prompt
• Fix: when directives are present but no evidence has been gathered, skip the text-response path and fall through to the final-prompt path which uses FINAL_SYSTEM_PROMPT (no directives) and says "No data was retrieved"

Test plan

• Integration test with real LLM: create bank with directive, no memories, reflect — assert directive text not in answer
• Verified red-green: test fails without fix, passes with fix
• All existing test_reflect_agent.py tests pass (no regressions)
• Lint passes