Task: Generate the handlers of an operation model

An operation model generates its NATS handler and registrar as an entity model does, so no operation handler is written by hand.

• An operation in an operation model names the service method it calls, the permission it checks and whether it requires a party, and codegen generates the handler that authorises, decodes, calls and replies.
• The registrar subscribing every operation is generated.
• The configuration document handlers in refdata, analytics and reporting, and the calendar materialisation handler, are generated and their hand-written copies deleted.

(Implementation strategy. Written when work starts; key decisions are distilled into the parent story's * Decisions at close, but the plan itself stays — it is the historical record of what we did.)

Every number this task claims about a tree, with the command that produced it and the commit it was measured at. A number without both is not a measurement, because a clean-up record describes a tree and trunk moves under a branch: a consumer count, a gate's component count and a suite's size all change with no code changing, and the reader of a bare number cannot tell which tree it came from. Where the raw output matters, put it in an evidence/ file beside the task and name that file in the Command column.

Raised on 2026-10-05 by the operator during the review of PR #2639: the configuration document operations generate their protocol from operation models, but their handlers and registrars are hand-written, as every operation handler is today, because the handler and registrar templates serve entity models only. The PR went in as it was; this task removes the hand-written handlers.

Manual QA scenarios (scaffolded via compass add test_scenario) that verify this task. Link new ones here as they're created; the scenario doc itself links back via its "Verifies task" field.