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>

ml/agents/overdue_task.py

@@ -0,0 +1,42 @@
+from __future__ import annotations
+from typing import ClassVar
+from .base import BaseAgent, AgentInput, AgentOutput
+
+
+class OverdueTaskAgent(BaseAgent):
+    """Reports the user's overdue tasks by count and age."""
+    agent_id: ClassVar[str] = "overdue-task"
+    ttl_seconds: ClassVar[int] = 3600  # 1h — overdue status changes infrequently
+    version: ClassVar[str] = "1.0.0"
+
+    def compute(self, inp: AgentInput) -> AgentOutput:
+        overdue = [t for t in inp.tasks if t.get("is_overdue")]
+        top = sorted(overdue, key=lambda t: -t.get("task_age_days", 0))[:3]
+
+        if not overdue:
+            prompt = "The user has no overdue tasks at this time."
+        elif len(overdue) == 1:
+            t = top[0]
+            age = round(t.get("task_age_days", 0))
+            prompt = (
+                f'The user has 1 overdue task: "{t["content"]}" '
+                f"({age} day{'s' if age != 1 else ''} overdue)."
+            )
+        else:
+            items = ", ".join(
+                f'"{t["content"]}" ({round(t.get("task_age_days", 0))}d)'
+                for t in top
+            )
+            prompt = (
+                f"The user has {len(overdue)} overdue tasks. "
+                f"Top {len(top)}: {items}."
+            )
+
+        snapshot = {
+            "overdue_count": len(overdue),
+            "top_overdue": [
+                {"content": t["content"], "task_age_days": t.get("task_age_days", 0)}
+                for t in top
+            ],
+        }
+        return self._make_output(inp, prompt, snapshot)