Portico agent-dispatch.yml missing agent_model input causes 100% dispatch 422 failures

feature_id: FEAT-DISPATCHER

## Root cause

On 2026-07-23 the dispatcher started sending agent_model as a workflow_dispatch input for every ticket (part of OPER-27 model-override work). Operant-studio agent-dispatch.yml was updated to declare that input. Portico agent-dispatch.yml was NOT so every POST to Operant-Health/portico/actions/workflows/agent-dispatch.yml/dispatches now returns HTTP 422 (Unexpected inputs provided).

## Evidence

- 2,045 of 2,046 workflow_dispatch_failed audit rows in the last 48h are HTTP 422 (source: DispatchAudit)
- POR-422 alone: 704 failed dispatches in 24h; POR-67: 690. Both auto-bounced 2026-07-24.

## Fix

Mirror operant-studio: add agent_model input to .github/workflows/agent-dispatch.yml and forward it to the reusable workflow.