fix(agents): instruct orchestrator to output tip in English

Small models (qwen2.5:1.5b) mirror the language of task title content
in the prompt. Adding an explicit English note to snippets that embed
raw task titles (focus-area, overdue-task) prevents language bleed.
Also added the instruction to the orchestrator system prompt and user
message as belt-and-suspenders.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ml/serving/prompts.py

@@ -116,6 +116,7 @@ _SYS_V4_ORCHESTRATOR = (
     "Multiple specialized agents have analyzed the user's current context and provided "
     "their insights below. Synthesize their combined perspective to generate exactly ONE "
     "tip that is specific, actionable, and relevant right now. "
+    "Always respond in English regardless of the language of task content. "
     "Respond ONLY with a JSON object with keys: "
     '"id" (short slug), "content" (the tip, ≤2 sentences), '
     '"rationale" (why now, ≤1 sentence). '
@@ -147,7 +148,7 @@ def build_orchestrator_messages(
         )
         for t in tasks[:3]:
             lines.append(f"  - {t.get('content', '?')}")
-    lines.append("\nGenerate one tip as a JSON object.")
+    lines.append("\nGenerate one tip as a JSON object. Write the tip content in English only.")
     return [
         {"role": "system", "content": _SYS_V4_ORCHESTRATOR},
         {"role": "user", "content": "\n".join(lines)},