switch from tags to digest for images

[peterj]

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[Copilot]

Pull request overview

This PR updates the controller build and agent translation logic so declarative agents use OCI digest-pinned runtime images (@sha256:...) rather than tags, enabling compatibility with Substrate ActorTemplate validation requirements.

Changes:

• Adds a build helper script to resolve agent runtime image digests from the registry and inject them into the controller binary via Go -ldflags -X.
• Updates the agent deployment translation path to emit digest-pinned runtime images and fail fast if required digests were not injected at link time.
• Updates golden test outputs and unit tests to validate digest-pinned image references.

Reviewed changes

Copilot reviewed 43 out of 43 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
scripts/controller-digest-ldflags.sh	New helper to inspect pushed image digests and emit -X ldflags for link-time injection.
Makefile	Adjusts LDFLAGS quoting and updates build-controller to build/push runtime images first, then embed their digests into the controller build.
go/README.md	Documents link-time embedding of agent runtime image digests and how make build-controller wires it up.
go/core/internal/controller/translator/agent/testdata/outputs/*.json	Updates golden outputs to use digest-pinned app@sha256:... references.
go/core/internal/controller/translator/agent/runtime_test.go	Updates runtime image assertions to validate digest-pinned references and adds helpers for setting digest globals in tests.
go/core/internal/controller/translator/agent/imageconfig_test.go	Adds unit coverage for digest pinning helpers and digest-required runtime resolution behavior.
go/core/internal/controller/translator/agent/digest_testmain_external_test.go	Sets default digest globals for external-package translator tests.
go/core/internal/controller/translator/agent/deployments.go	Introduces digest-based runtime image resolution and removes tag-based runtime selection.
go/core/internal/controller/translator/agent/adk_api_translator.go	Extends ImageConfig with Digest, adds digest normalization + pinning helper, and adds link-time digest globals.

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