ContainerConfig.image is now Option<String>, mutually exclusive with a new
optional ContainerConfig.build: Option<BuildConfig>. Exactly one of image
or build must be present, enforced in AppManifest::validate.
Adds ResolvedSource enum (Pull | Build) and ContainerConfig::resolve +
::image_ref helpers so the orchestrator can treat pull and build uniformly.
All 26 existing pull-only manifests continue to parse unchanged
(covered by existing_pull_only_manifests_still_parse test).
Call sites updated: podman_client, runtime::DockerRuntime, dev_orchestrator.
Dev orchestrator errors out cleanly on Build sources until Step 2 lands
build_image support on the runtime trait.
Step 1 of docs/rust-orchestrator-migration.md. 10 new unit tests, all pass.
Also includes: docs/rust-orchestrator-migration.md (design spec) and
docs/STATUS.md resume section for the next session.
