feat: ExpressDispatchAgent — tenant-scoped batch assignment + sequencing

New agent that owns the DoormileExpress dispatch flow. The normal B2C booking
flow is untouched; this handles express orders that a console operator has let
accumulate batch by batch, then dispatches in one go.

Triggered by express.dispatch_requested (the console's manual dispatch action).
For the tenant it: reads the pending bookings and the tenant's available riders
(Go internal API), distributes them across riders by proximity + load with a
radius guard, asks routes.workolik.com (Valhalla) for each rider's stop order,
and writes the assignments back through the Go internal API — which stays the
single writer of assignment state; the agent only decides who and what order.

- agents/express_dispatch_agent.py: the agent (mirrors DispatchAgent's NATS
  binding; pure consumer of the backend-owned EXPRESS stream).
- config/system_config.py: ROUTE_OPTIMIZER_URL.
- registered under JARVIS in main.py / agents package.
- tests: distribution (radius, load, cap, no-GPS fallback) and sequencing
  (single-stop, remap, optimizer-down, unknown-id) — deterministic, no network.

Gated off by default on the backend (EXPRESS_AGENT_ENABLED); EXPRESS_AGENT_
AUTONOMOUS=false runs it observe-only (logs the plan, writes nothing).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

agents/__init__.py

@@ -6,6 +6,7 @@ from agents.hub_agent import HubAgent
 from agents.customer_agent import CustomerAgent
 from agents.exception_agent import ExceptionAgent
 from agents.route_optimizer_agent import RouteOptimizerAgent
+from agents.express_dispatch_agent import ExpressDispatchAgent
 
 __all__ = [
     'OrderAgent',
@@ -14,5 +15,6 @@ __all__ = [
     'HubAgent',
     'CustomerAgent',
     'ExceptionAgent',
-    'RouteOptimizerAgent'
+    'RouteOptimizerAgent',
+    'ExpressDispatchAgent'
 ]