feat(ml): multi-agent context framework + v4 orchestrator prompt

Adds ml/agents/ — five specialised sub-agents (overdue_task, momentum,
time_of_day, recent_patterns, focus_area) each producing a prompt snippet
from user signals. A registry wires them up; the orchestrator prompt in
ml/serving/prompts.py synthesises their outputs into one tip via LiteLLM.

Also wires /api/agents route in the API and updates the Dockerfile to copy
the full ml/ tree with PYTHONPATH=/app so agent imports resolve correctly.

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

infra/docker/Dockerfile.ml

@@ -1,6 +1,8 @@
 FROM python:3.12-slim
-WORKDIR /app
+WORKDIR /app/ml/serving
 COPY ml/serving/requirements.txt .
 RUN pip install --no-cache-dir -r requirements.txt
-COPY ml/serving/*.py .
+COPY ml/ /app/ml/
+# PYTHONPATH=/app lets 'import ml.agents.*' resolve from /app/ml/agents/
+ENV PYTHONPATH=/app
 CMD ["uvicorn", "main:app", "--host", "0.0.0.0", "--port", "8000"]