* feat(ecs): ecs-agent skeleton + containerd image pull (Sprint 4c)
Add the ecs-agent binary that runs inside an ECS container instance: it
resolves host identity from IMDS, registers on the Layer-2 NATS bus, and
heartbeats while alive. Image pulls go through a containerd-backed
ImagePuller behind an interface, with an ECR resolver that mints
GetAuthorizationToken creds from the instance role.
Extract the IMDS-credential and ECR-auth flow out of
ecr-credential-provider into internal/imdscreds and internal/ecrauth so
the agent and the kubelet provider share one implementation.
Add spinifex/handlers/ecs/bus as the single source of truth for Layer-2
subject builders and wire payloads, shared by the agent now and the
scheduler in Sprint 4e.
* build: pin genproto via replace to fix containerd/v2 ambiguous import
go.opencensus.io -> grpc@v1.33.2 drags the pre-split monolithic genproto
into the module graph; it still ships googleapis/rpc/status, which collides
with the split genproto/googleapis/rpc module that containerd/ttrpc needs.
The main module never imports the monolith, so a plain require bump is
dropped by 'go mod tidy'. Pin it with a replace to a post-split version so
the fix survives tidy and contributors can run 'go mod tidy' freely.
* fix(ecs): agent boot failsafes from runtime verify
- Build the ECR gateway HTTP client lazily on first image pull instead of
at boot. A missing/malformed gateway CA no longer kills the agent before
it can register + heartbeat, matching the best-effort containerd path.
- Actively probe containerd with IsServing in runtime.New. The client dials
lazily, so a dead socket previously slipped past the boot-time
'containerd unavailable' warn and only surfaced on the first pull.
- Make the heartbeat interval configurable via ECS_HEARTBEAT_INTERVAL
(default 30s) for test/verify and slow-link tuning.
Julian Sommer