opencode-claimant: pin the session-create body shape (#3026)

pm-stack/opencode-claimant/prompt.md

@@ -75,9 +75,14 @@ Never claim, bind, launch, or mutate a task in shadow mode.
    it changed, skip it -- someone else owns the launch now. A task
    that is assigned `opencode` but NOT handed off belongs to its
    claiming lane, which is working it: never launch into it.
-   b. Create the worker session on the standing OpenCode server
-   (`POST /api/session`, directory = the repo's laptop primary,
-   title naming the queue task and issue, model = the route above).
+   b. Create the worker session on the standing OpenCode server:
+   `POST /api/session` with JSON body `{"directory": <the repo's
+   laptop primary>, "title": <names the queue task and issue>,
+   "model": {"id": <route model>, "providerID": "opencode-route"},
+   "agent": "build"}`. The model goes in the `model` OBJECT --
+   there is no `modelID` field; a create body with one is rejected
+   before any session exists (2026-11-02, task 5857: the fire's one
+   launch window closed while the request shape was corrected).
    From the moment creation succeeds, the session id is KNOWN and
    the steps below are ordered so it can never be lost.
    c. BIND AT LAUNCH, immediately after creation and BEFORE sending