feat(agents): manifest plumbing + GET /agents/registry (ADR-0014 step 3)

Each agent now exports a module-level MANIFEST declaring id, version,
pref_schema, required_consents, ttl_sec, and silenced_in_contexts. The
registry surfaces both the agent and its manifest, and rejects on
mismatch so the two cannot drift.

ml/serving exposes GET /agents/registry; services/api proxies it as
GET /api/agents/registry with a 60s in-process cache so admin pageviews
don't hammer upstream. Failures aren't cached.

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

ml/agents/overdue_task.py

@@ -1,13 +1,38 @@
 from __future__ import annotations
 from typing import ClassVar
 from .base import BaseAgent, AgentInput, AgentOutput
+from .manifest import AgentManifest
+
+
+MANIFEST = AgentManifest(
+    id="overdue-task",
+    version="1.0.0",
+    description="Reports the user's overdue tasks by count and age.",
+    pref_schema={
+        "type": "object",
+        "additionalProperties": False,
+        "properties": {
+            "lateness_tolerance_days": {
+                "type": "integer",
+                "minimum": 0,
+                "default": 0,
+                "description": "Days past due before a task is considered overdue. 0 = the moment it's late.",
+            },
+        },
+    },
+    context_schema=["todoist.tasks"],
+    required_consents=["data:core", "data:todoist", "agent:overdue-task"],
+    output_contract={"type": "snippet", "format": "free_text"},
+    ttl_sec=3600,
+    silenced_in_contexts=["vacation"],
+)
 
 
 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"
+    agent_id: ClassVar[str] = MANIFEST.id
+    ttl_seconds: ClassVar[int] = MANIFEST.ttl_sec
+    version: ClassVar[str] = MANIFEST.version
 
     def compute(self, inp: AgentInput) -> AgentOutput:
         overdue = [t for t in inp.tasks if t.get("is_overdue")]