Add BASE_PROMPT: self-contained responses, no speculation

Minimal base prompt for the explorer — no persona, just quality
and guardrails.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app.py

@@ -41,6 +41,12 @@ HF_TOKEN = os.environ.get("HF_TOKEN", "")
 
 SOURCE = "prod" if os.environ.get("SPACE_ID") else "dev"
 
+BASE_PROMPT = (
+    "Your response should stand on its own.\n\n"
+    "Do not speculate, manufacture connections to make a question fit, or answer "
+    "off-topic questions."
+)
+
 hf_api = HfApi(token=HF_TOKEN) if HF_TOKEN else None
 
 
@@ -176,7 +182,7 @@ def stream_question(
         scratch_path = tempfile.mkdtemp(prefix="lh-scratch-")
     scratch_dir = Path(scratch_path)
 
-    system_prompt = build_system_prompt(base_prompt="", workspace=workspace)
+    system_prompt = build_system_prompt(base_prompt=BASE_PROMPT, workspace=workspace)
     messages: list[Message] = [
         {"role": "system", "content": system_prompt},
         {"role": "user", "content": question},