What this PR does / why we need it:

Introduce jobs top-level element and a layered container specification

Motivation

Services are long-running processes, but Compose users also need to define one-shot tasks — database migrations, batch processing, scheduled cleanups. A common hack is to declare those as "services" with a profile so they are excluded by default, and invoke them with docker compose run.
This PR introduces jobs as a first-class top-level element in the Compose specification to address this gap.

To avoid duplicating the full container configuration schema between element types, the shared configuration is extracted as a layered specification:

• container_spec — the attributes of a single container, shared by anything that runs one: services, jobs, and run-to-completion init containers (pre_start hooks);
• workload_spec — orchestrated-workload attributes (build, depends_on, expose, healthcheck, ports, stdin_open, tty), shared by services and jobs;
• service-only attributes (attach, container_name, restart, scale, deploy, develop, lifecycle hooks, …) stay in the services document.

As a direct consequence, pre_start hooks become a full container specification (allOf [container_spec] + per_replica) instead of a hand-curated 7-attribute subset: undeclared attributes are inherited from the service, declared ones override with the merge-file rules — the direction proposed in #656. Attributes excluded from inheritance by that proposal (ports, healthcheck, depends_on, build, …) are excluded by construction: they live in workload_spec, which hooks do not compose.

jobs:
    db-migration:
      image: myapp:latest
      command: python manage.py migrate
      triggers:
        manual: true

    cleanup:
      image: busybox
      command: sh -c 'find /data -mtime +30 -delete'
      triggers:
        schedule: "0 3 * * *"

Note: this requires JSON Schema draft 2020-12 as proposed on compose-spec/compose-go#864 so we can use unevaluatedProperties for composition by allOf/$ref

Which issue(s) this PR fixes:
Fixes #92
Addresses #656 (schema side)