Automation definitions: validation, storage and the wire shape.

An automation joins triggers, one prompt, one target and delivery options.
The runner owns execution; this namespace never starts a turn. Definitions
keep the contract spelling: string keys in snake_case.

Five-field cron expressions and their fire times in one time zone.

A spec matches minute, hour, day of month, month and day of week. Day 0 and
day 7 are Sunday. When both day fields are restricted, a day matches either
field, as in Vixie cron. A local time in a daylight-saving gap fires at the
end of the gap. A local time that occurs twice fires only at the first one.

Python host surface for automations. Results never hold a webhook or callback
secret: a person creates secrets in the Companion app or the TUI.

Run automations: the scheduler, the run queue, webhooks and delivery.

The gateway fills the runtime through `install-runtime!`, so this namespace
never requires the gateway. A run row is the claim: one trigger occurrence
runs at most once. Runs of one automation run one after another. A queued or
running run of a stopped gateway becomes unknown and never runs again.

Webhook checks: signatures, event names, filters and prompt templates.

Every function here is pure. A payload is untrusted data: it can fill a
prompt template, but it never selects a target, a model or a tool.